Slicers (applies to each fact value in each table cell)

Reporting Entity [Axis]

0000005513 (http://www.sec.gov/CIK)

Notes to Financial Statements [Abstract]	Period [Axis]
	2016-01-01 - 2016-12-31
Notes to Financial Statements [Abstract]
Amortized Cost and Fair Values of Securities by Security Type	At December 31, 2016 and 2015, all fixed maturity securities were classified as available-for-sale. The amortized cost and fair values of securities by security type are shown as follows:   December 31, 2016   Amortized Cost   Gross Unrealized Gain   Gross Unrealized Loss   Fair Value   (in millions of dollars) United States Government and Government Agencies and Authorities $ 1,202.8   $ 183.1   $ 3.5   $ 1,382.4 States, Municipalities, and Political Subdivisions 1,868.0   294.8   4.8   2,158.0 Foreign Governments 714.8   199.9   —   914.7 Public Utilities 6,916.1   1,123.5   16.9   8,022.7 Mortgage/Asset-Backed Securities 2,104.9   134.7   9.2   2,230.4 All Other Corporate Bonds 26,707.1   2,944.0   183.9   29,467.2 Redeemable Preferred Stocks 39.0   3.2   0.3   41.9 Total Fixed Maturity Securities $ 39,552.7   $ 4,883.2   $ 218.6   $ 44,217.3   December 31, 2015   Amortized Cost   Gross Unrealized Gain   Gross Unrealized Loss   Fair Value   (in millions of dollars) United States Government and Government Agencies and Authorities $ 1,265.8   $ 207.3   $ 7.9   $ 1,465.2 States, Municipalities, and Political Subdivisions 1,828.3   293.4   4.6   2,117.1 Foreign Governments 897.2   154.4   —   1,051.6 Public Utilities 6,979.3   1,057.4   16.3   8,020.4 Mortgage/Asset-Backed Securities 2,318.6   167.6   4.7   2,481.5 All Other Corporate Bonds 26,325.5   2,454.1   608.2   28,171.4 Redeemable Preferred Stocks 44.0   3.8   0.6   47.2 Total Fixed Maturity Securities $ 39,658.7   $ 4,338.0   $ 642.3   $ 43,354.4
Length of Time Fixed Maturity Securities had been in a Gross Unrealized Loss Position	The following charts indicate the length of time our fixed maturity securities have been in a gross unrealized loss position.   December 31, 2016   Less Than 12 Months   12 Months or Greater   Fair Value   Gross Unrealized Loss   Fair Value   Gross Unrealized Loss   (in millions of dollars) United States Government and Government Agencies and Authorities $ 132.8   $ 3.5   $ —   $ — States, Municipalities, and Political Subdivisions 132.2   4.8   —   — Public Utilities 260.2   15.3   15.6   1.6 Mortgage/Asset-Backed Securities 513.2   9.1   0.8   0.1 All Other Corporate Bonds 3,621.0   122.1   774.7   61.8 Redeemable Preferred Stocks 7.9   0.1   10.8   0.2 Total Fixed Maturity Securities $ 4,667.3   $ 154.9   $ 801.9   $ 63.7   December 31, 2015   Less Than 12 Months   12 Months or Greater   Fair Value   Gross Unrealized Loss   Fair Value   Gross Unrealized Loss   (in millions of dollars) United States Government and Government Agencies and Authorities $ 213.5   $ 7.9   $ —   $ — States, Municipalities, and Political Subdivisions 112.3   4.3   3.3   0.3 Public Utilities 408.4   14.4   10.3   1.9 Mortgage/Asset-Backed Securities 504.3   4.6   9.0   0.1 All Other Corporate Bonds 6,155.0   464.2   554.7   144.0 Redeemable Preferred Stocks 10.4   0.6   —   — Total Fixed Maturity Securities $ 7,403.9   $ 496.0   $ 577.3   $ 146.3
Distribution of the Maturity Dates for Fixed Maturity Securities	The following is a distribution of the maturity dates for fixed maturity securities. The maturity dates have not been adjusted for possible calls or prepayments.   December 31, 2016   Total Amortized Cost   Unrealized Gain Position   Unrealized Loss Position     Gross Gain   Fair Value   Gross Loss   Fair Value   (in millions of dollars) 1 year or less $ 1,338.8   $ 28.6   $ 1,355.6   $ —   $ 11.8 Over 1 year through 5 years 6,231.0   553.5   6,605.6   8.2   170.7 Over 5 years through 10 years 10,991.6   843.8   9,336.2   82.8   2,416.4 Over 10 years 18,886.4   3,322.6   19,734.3   118.4   2,356.3   37,447.8   4,748.5   37,031.7   209.4   4,955.2 Mortgage/Asset-Backed Securities 2,104.9   134.7   1,716.4   9.2   514.0 Total Fixed Maturity Securities $ 39,552.7   $ 4,883.2   $ 38,748.1   $ 218.6   $ 5,469.2   December 31, 2015   Total Amortized Cost   Unrealized Gain Position   Unrealized Loss Position     Gross Gain   Fair Value   Gross Loss   Fair Value   (in millions of dollars) 1 year or less $ 1,112.2   $ 20.6   $ 1,098.8   $ 0.2   $ 33.8 Over 1 year through 5 years 6,514.0   554.4   6,649.5   49.8   369.1 Over 5 years through 10 years 10,519.3   746.3   7,124.4   320.5   3,820.7 Over 10 years 19,194.6   2,849.1   18,532.3   267.1   3,244.3   37,340.1   4,170.4   33,405.0   637.6   7,467.9 Mortgage/Asset-Backed Securities 2,318.6   167.6   1,968.2   4.7   513.3 Total Fixed Maturity Securities $ 39,658.7   $ 4,338.0   $ 35,373.2   $ 642.3   $ 7,981.2
Low Income Housing Tax Credits	The Company invests in tax credit partnerships primarily for the receipt of income tax credits and tax benefits derived from passive losses on the investments. Amounts recognized in the consolidated statements of income are as follows:   Year Ended December 31   2016   2015   2014   (in millions of dollars) Income Tax Credits $ 41.8   $ 41.8   $ 41.8 Amortization, Net of Tax (23.2 )   (23.3 )   (23.2 ) Income Tax Benefit $ 18.6   $ 18.5   $ 18.6
Mortgage Loans by Property Type and Geographic Region	Mortgage loans by property type and geographic region are presented below.   December 31   2016   2015   (in millions of dollars)   Carrying   Percent of   Carrying   Percent of   Amount   Total   Amount   Total Property Type                    Apartment $ 288.4   14.1 %   $ 130.6   6.9 %      Industrial 573.6   28.1   574.1   30.5      Office 700.1   34.4   764.7   40.6      Retail 455.4   22.4   392.3   20.8 Other 21.4   1.0   21.9   1.2 Total $ 2,038.9   100.0 %   $ 1,883.6   100.0 % Region                    New England $ 72.7   3.6 %   $ 97.6   5.2 %      Mid-Atlantic 125.3   6.1   128.8   6.9      East North Central 230.1   11.3   186.4   9.9      West North Central 172.0   8.4   162.6   8.6      South Atlantic 438.3   21.5   409.3   21.7      East South Central 91.6   4.5   79.1   4.2      West South Central 268.7   13.2   237.6   12.6      Mountain 214.1   10.5   196.5   10.4      Pacific 426.1   20.9   385.7   20.5 Total $ 2,038.9   100.0 %   $ 1,883.6   100.0 %
Mortgage Loans by Credit Quality Indicators	Mortgage loans, sorted by the applicable credit quality indicators, are as follows:   December 31   2016   2015   (in millions of dollars) Internal Rating            Aa $ 0.7   $ 1.1      A 488.2   586.6      Baa 1,506.6   1,285.8      Ba 43.4   10.1 Total $ 2,038.9   $ 1,883.6 Loan-to-Value Ratio            <= 65% $ 917.9   $ 937.2      > 65% <= 75% 1,011.5   842.5      > 75% <= 85% 50.8   88.4      > 85% 58.7   15.5 Total $ 2,038.9   $ 1,883.6
Troubled Debt Restructurings on Financing Receivables	A summary of our troubled debt restructurings is as follows:   Year Ended December 31   2016   2015   2014   (in millions of dollars) Foreclosure           Carrying Amount $ 5.4   $ —   $ 18.1 Number of Loans 1   —   1
Activity in the Allowance for Credit Losses	The activity in the allowance for credit losses is as follows:   Year Ended December 31   2016   2015   2014   (in millions of dollars) Balance at Beginning of Year $ —   $ 1.5   $ 1.5 Provision —   0.5   3.0 Charge-offs, Net of Recoveries —   (2.0 )   (3.0 ) Balance at End of Year $ —   $ —   $ 1.5
Remaining Contractual Maturity of Securities Lending Agreements	The remaining contractual maturities of our securities lending agreements disaggregated by class of collateral pledged are as follows:   December 31   2016   2015   Overnight and Continuous   (in millions of dollars) United States Government and Government Agencies and Authorities $ 0.1   $ 1.2 Public Utilities 0.1   4.0 All Other Corporate Bonds 29.7   23.8 Total Borrowings 29.9   29.0 Gross Amount of Recognized Liability for Securities Lending Transactions 29.9   29.0 Amounts Related to Agreements Not Included in Offsetting Disclosure Contained Herein $ —   $ —
Schedule of Financial Instrument and Derivative Offsetting	Shown below are our financial instruments that either meet the accounting requirements that allow them to be offset in our balance sheets or that are subject to an enforceable master netting arrangement or similar agreement. Our accounting policy is to not offset these financial instruments in our balance sheets. Net amounts disclosed below have been reduced by the amount of collateral pledged to or received from our counterparties.     December 31, 2016     Gross Amount           Gross Amount Not         of Recognized   Gross Amount   Net Amount   Offset in Balance Sheet         Financial   Offset in   Presented in   Financial   Cash   Net     Instruments   Balance Sheet   Balance Sheet   Instruments   Collateral   Amount     (in millions of dollars) Financial Assets:   Derivatives   $ 32.7   $ —   $ 32.7   $ (7.3 )   $ (25.4 )   $ — Securities Lending   178.5   —   178.5   (148.6 )   (29.9 )   — Total   $ 211.2   $ —   $ 211.2   $ (155.9 )   $ (55.3 )   $ —       Financial Liabilities:                         Derivatives   $ 52.8   $ —   $ 52.8   $ (37.6 )   $ —   $ 15.2 Securities Lending   29.9   —   29.9   (29.9 )   —   — Total   $ 82.7   $ —   $ 82.7   $ (67.5 )   $ —   $ 15.2     December 31, 2015     Gross Amount           Gross Amount Not         of Recognized   Gross Amount   Net Amount   Offset in Balance Sheet         Financial   Offset in   Presented in   Financial   Cash   Net     Instruments   Balance Sheet   Balance Sheet   Instruments   Collateral   Amount     (in millions of dollars) Financial Assets:     Derivatives   $ 49.8   $ —   $ 49.8   $ (12.8 )   $ (36.4 )   $ 0.6 Securities Lending   181.6   —   181.6   (152.6 )   (29.0 )   — Total   $ 231.4   $ —   $ 231.4   $ (165.4 )   $ (65.4 )   $ 0.6                           Financial Liabilities:                         Derivatives   $ 50.2   $ —   $ 50.2   $ (35.6 )   $ —   $ 14.6 Securities Lending   29.0   —   29.0   (29.0 )   —   — Total   $ 79.2   $ —   $ 79.2   $ (64.6 )   $ —   $ 14.6
Net Investment Income	Net Investment Income Net investment income reported in our consolidated statements of income is as follows:   Year Ended December 31   2016   2015   2014   (in millions of dollars) Fixed Maturity Securities $ 2,293.9   $ 2,327.1   $ 2,344.4 Derivatives 49.8   44.2   40.4 Mortgage Loans 114.0   114.0   109.8 Policy Loans 17.3   16.7   16.3 Other Long-term Investments 22.6   21.6   23.0 Short-term Investments 7.5   3.4   2.4 Gross Investment Income 2,505.1   2,527.0   2,536.3 Less Investment Expenses 31.9   31.2   29.0 Less Investment Income on Participation Fund Account Assets 14.2   14.6   15.1 Net Investment Income $ 2,459.0   $ 2,481.2   $ 2,492.2
Realized Investment Gains and Losses Reported in Consolidated Statements of Income	Realized Investment Gain and Loss Realized investment gains and losses are as follows:   Year Ended December 31   2016   2015   2014   (in millions of dollars) Fixed Maturity Securities           Gross Gains on Sales $ 34.3   $ 23.8   $ 9.3 Gross Losses on Sales (17.2 )   (25.6 )   (7.5 ) Other-Than-Temporary Impairment Loss (30.5 )   (32.4 )   (13.5 ) Mortgage Loans and Other Invested Assets           Gross Gains on Sales 5.5   16.0   21.2 Gross Losses on Sales (0.7 )   (0.1 )   (0.8 ) Impairment Loss (5.7 )   (5.9 )   (3.4 ) Embedded Derivative in Modified Coinsurance Arrangement 40.9   (37.7 )   3.3 All Other Derivatives (1.5 )   35.7   11.0 Foreign Currency Transactions (0.9 )   (17.6 )   (3.5 ) Net Realized Investment Gain (Loss) $ 24.2   $ (43.8 )   $ 16.1