| Fair Value Disclosures [Abstract] | Period [Axis] |
|---|
2016-01-01 - 2016-12-31 |
|---|
Fair Value Disclosures [Abstract] | |
Fair Value Measurements By Level For Assets Measured At Fair Value On A Recurring Basis | A summary of fair value measurements by level for assets and liabilities measured at fair value on a recurring basis at December 31, 2016 and 2015 is as follows: | | | | | | | | | | | | | | | | | | Level I | | Level II | | Level III | | Total | December 31, 2016 | | | | | | | | Assets: | | | | | | | | Cash equivalents | $ | 1,546.0 |
| | $ | — |
| | $ | — |
| | $ | 1,546.0 |
| Investments available-for-sale: |
| |
| |
| | | Fixed maturity securities: |
| |
| |
| | | United States Government securities | 558.5 |
| | — |
| | — |
| | 558.5 |
| Government sponsored securities | — |
| | 40.0 |
| | — |
| | 40.0 |
| States, municipalities and political subdivisions, tax-exempt | — |
| | 6,105.3 |
| | — |
| | 6,105.3 |
| Corporate securities | 79.9 |
| | 7,775.9 |
| | 238.8 |
| | 8,094.6 |
| Residential mortgage-backed securities | — |
| | 1,917.3 |
| | 12.0 |
| | 1,929.3 |
| Commercial mortgage-backed securities | — |
| | 214.3 |
| | — |
| | 214.3 |
| Other debt securities | 53.4 |
| | 649.3 |
| | 42.8 |
| | 745.5 |
| Total fixed maturity securities | 691.8 |
| | 16,702.1 |
| | 293.6 |
| | 17,687.5 |
| Equity securities | 1,200.2 |
| | 111.9 |
| | 187.8 |
| | 1,499.9 |
| Other invested assets, current | 15.8 |
| | — |
| | — |
| | 15.8 |
| Securities lending collateral | 726.0 |
| | 353.8 |
| | — |
| | 1,079.8 |
| Derivatives | — |
| | 758.7 |
| | — |
| | 758.7 |
| Total assets | $ | 4,179.8 |
| | $ | 17,926.5 |
| | $ | 481.4 |
| | $ | 22,587.7 |
| Liabilities: | | | | | | | | Derivatives | $ | — |
|
| $ | (241.9 | ) |
| $ | — |
|
| $ | (241.9 | ) | Total liabilities | $ | — |
| | $ | (241.9 | ) | | $ | — |
| | $ | (241.9 | ) | | | | | | | | | December 31, 2015 | | | | | | | | Assets: | | | | | | | | Cash equivalents | $ | 701.0 |
| | $ | — |
| | $ | — |
| | $ | 701.0 |
| Investments available-for-sale: | | | | | | | | Fixed maturity securities: | | | | | | | | United States Government securities | 349.9 |
| | — |
| | — |
| | 349.9 |
| Government sponsored securities | — |
| | 75.9 |
| | — |
| | 75.9 |
| States, municipalities and political subdivisions, tax-exempt | — |
| | 6,251.6 |
| | — |
| | 6,251.6 |
| Corporate securities | 77.6 |
| | 7,629.3 |
| | 186.2 |
| | 7,893.1 |
| Residential mortgage-backed securities | — |
| | 1,750.9 |
| | — |
| | 1,750.9 |
| Commercial mortgage-backed securities | — |
| | 402.4 |
| | 1.9 |
| | 404.3 |
| Other debt securities | 55.7 |
| | 671.2 |
| | 25.6 |
| | 752.5 |
| Total fixed maturity securities | 483.2 |
| | 16,781.3 |
| | 213.7 |
| | 17,478.2 |
| Equity securities | 1,253.8 |
| | 116.9 |
| | 102.1 |
| | 1,472.8 |
| Other invested assets, current | 19.1 |
| | — |
| | — |
| | 19.1 |
| Securities lending collateral | 708.1 |
| | 592.3 |
| | — |
| | 1,300.4 |
| Derivatives | — |
| | 329.7 |
| | — |
| | 329.7 |
| Total assets | $ | 3,165.2 |
| | $ | 17,820.2 |
| | $ | 315.8 |
| | $ | 21,301.2 |
| Liabilities: | | | | | | | | Derivatives | $ | — |
| | $ | (430.0 | ) | | $ | — |
| | $ | (430.0 | ) | Total liabilities | $ | — |
| | $ | (430.0 | ) | | $ | — |
| | $ | (430.0 | ) |
|
Reconciliation Of The Beginning And Ending Balances Of Assets Measured At Fair Value On A Recurring Basis Using Level III Inputs | A reconciliation of the beginning and ending balances of assets measured at fair value on a recurring basis using Level III inputs for the years ended December 31, 2016, 2015 and 2014 is as follows: | | | | | | | | | | | | | | | | | | | | | | | | | | Corporate Securities | | Residential Mortgage- backed Securities | | Commercial Mortgage- backed Securities | | Other Debt Securities | | Equity Securities | | Total | Year ended December 31, 2016 | | | | | | | | | | | | Beginning balance at January 1, 2016 | $ | 186.2 |
| | $ | — |
| | $ | 1.9 |
| | $ | 25.6 |
| | $ | 102.1 |
| | $ | 315.8 |
| Total (losses) gains: | | | | | | | | | | | | Recognized in net income | (2.9 | ) | | — |
| | — |
| | — |
| | 0.7 |
| | (2.2 | ) | Recognized in accumulated other comprehensive income | (2.0 | ) | | — |
| | — |
| | (0.5 | ) | | (0.5 | ) | | (3.0 | ) | Purchases | 170.2 |
| | 4.3 |
| | — |
| | — |
| | 222.6 |
| | 397.1 |
| Sales | (5.4 | ) | | — |
| | — |
| | — |
| | (136.7 | ) | | (142.1 | ) | Settlements | (56.8 | ) | | — |
| | — |
| | (0.9 | ) | | (0.4 | ) | | (58.1 | ) | Transfers into Level III | 6.6 |
| | 9.3 |
| | — |
| | 28.8 |
| | — |
| | 44.7 |
| Transfers out of Level III | (57.1 | ) | | (1.6 | ) | | (1.9 | ) | | (10.2 | ) | | — |
| | (70.8 | ) | Ending balance at December 31, 2016 | $ | 238.8 |
| | $ | 12.0 |
| | $ | — |
| | $ | 42.8 |
| | $ | 187.8 |
| | $ | 481.4 |
| Change in unrealized losses included in net income related to assets still held for the year ended December 31, 2016 | $ | (2.0 | ) | | $ | — |
| | $ | — |
| | $ | — |
| | $ | — |
| | $ | (2.0 | ) | | | | | | | | | | | | | Year ended December 31, 2015 | | | | | | | | | | | | Beginning balance at January 1, 2015 | $ | 144.6 |
| | $ | — |
| | $ | 3.3 |
| | $ | 6.6 |
| | $ | 48.3 |
| | $ | 202.8 |
| Total gains (losses): | | | | | | | | | | | | Recognized in net income | 1.4 |
| | — |
| | — |
| | 0.2 |
| | (1.5 | ) | | 0.1 |
| Recognized in accumulated other comprehensive income | 0.7 |
| | — |
| | — |
| | (0.2 | ) | | 3.9 |
| | 4.4 |
| Purchases | 132.6 |
| | — |
| | 1.1 |
| | 28.3 |
| | 52.1 |
| | 214.1 |
| Sales | (11.7 | ) | | — |
| | (1.1 | ) | | (0.9 | ) | | (13.8 | ) | | (27.5 | ) | Settlements | (51.6 | ) | | — |
| | (1.4 | ) | | (0.2 | ) | | — |
| | (53.2 | ) | Transfers into Level III | 4.8 |
| | — |
| | — |
| | — |
| | 13.1 |
| | 17.9 |
| Transfers out of Level III | (34.6 | ) | | — |
| | — |
| | (8.2 | ) | | — |
| | (42.8 | ) | Ending balance at December 31, 2015 | $ | 186.2 |
| | $ | — |
| | $ | 1.9 |
| | $ | 25.6 |
| | $ | 102.1 |
| | $ | 315.8 |
| Change in unrealized losses included in net income related to assets still held for the year ended December 31, 2015 | $ | (0.6 | ) | | $ | — |
| | $ | — |
| | $ | — |
| | $ | (1.4 | ) | | $ | (2.0 | ) | | | | | | | | | | | | | Year ended December 31, 2014 | | | | | | | | | | | | Beginning balance at January 1, 2014 | $ | 115.2 |
| | $ | — |
| | $ | 6.5 |
| | $ | 14.8 |
| | $ | 41.4 |
| | $ | 177.9 |
| Total (losses) gains: | | | | | | | | | | | | Recognized in net income | (4.4 | ) | | — |
| | — |
| | — |
| | (0.7 | ) | | (5.1 | ) | Recognized in accumulated other comprehensive income | 8.5 |
| | — |
| | — |
| | 0.4 |
| | 2.8 |
| | 11.7 |
| Purchases | 68.9 |
| | — |
| | 3.6 |
| | 6.5 |
| | 15.9 |
| | 94.9 |
| Sales | (48.0 | ) | | — |
| | — |
| | (3.6 | ) | | (10.6 | ) | | (62.2 | ) | Settlements | (11.0 | ) | | — |
| | (3.7 | ) | | (0.4 | ) | | — |
| | (15.1 | ) | Transfers into Level III | 24.8 |
|
| — |
|
| — |
|
| — |
|
| — |
| | 24.8 |
| Transfers out of Level III | (9.4 | ) | | — |
| | (3.1 | ) | | (11.1 | ) | | (0.5 | ) | | (24.1 | ) | Ending balance at December 31, 2014 | $ | 144.6 |
| | $ | — |
| | $ | 3.3 |
| | $ | 6.6 |
| | $ | 48.3 |
| | $ | 202.8 |
| Change in unrealized losses included in net income related to assets still held for the year ended December 31, 2014 | $ | (11.1 | ) | | $ | — |
| | $ | — |
| | $ | — |
| | $ | (0.7 | ) | | $ | (11.8 | ) |
|
Carrying And Fair Values By Level Of Financial Instruments Not Recorded At Fair Value On Consolidated Balance Sheet | A summary of the estimated fair values by level of each class of financial instrument that is recorded at its carrying value on our consolidated balance sheets at December 31, 2016 and 2015 are as follows: | | | | | | | | | | | | | | | | | | | | | | Carrying Value | | Estimated Fair Value | | Level I | | Level II | | Level III | | Total | December 31, 2016 | | | | | | | | | | Assets: | | | | | | | | | | Other invested assets, long-term | $ | 2,240.5 |
| | $ | — |
| | $ | — |
| | $ | 2,240.5 |
| | $ | 2,240.5 |
| Liabilities: | | | | | | | | | | Debt: | | | | | | | | | | Short-term borrowings | 440.0 |
| | — |
| | 440.0 |
| | — |
| | 440.0 |
| Commercial paper | 629.0 |
| | — |
| | 629.0 |
| | — |
| | 629.0 |
| Notes | 14,323.8 |
| | — |
| | 14,858.4 |
| | — |
| | 14,858.4 |
| Convertible debentures | 334.1 |
| | — |
| | 1,020.2 |
| | — |
| | 1,020.2 |
| | | | | | | | | | | December 31, 2015 | | | | | | | | | | Assets: | | | | | | | | | | Other invested assets, long-term | $ | 2,041.1 |
| | $ | — |
| | $ | — |
| | $ | 2,041.1 |
| | $ | 2,041.1 |
| Liabilities: | | | | | | | | | | Debt: | | | | | | | | | | Short-term borrowings | 540.0 |
| | — |
| | 540.0 |
| | — |
| | 540.0 |
| Commercial paper | 682.2 |
| | — |
| | 682.2 |
| | — |
| | 682.2 |
| Notes | 14,311.6 |
| | — |
| | 14,523.2 |
| | — |
| | 14,523.2 |
| Convertible debentures | 330.7 |
| | — |
| | 980.1 |
| | — |
| | 980.1 |
|
|