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

Reporting Entity [Axis]

0001336047 (http://www.sec.gov/CIK)

Compensation and Retirement Disclosure [Abstract]	Period [Axis]
	2016-01-01 - 2016-12-31
Compensation and Retirement Disclosure [Abstract]
Projected Benefit Obligation, Fair Value of Assets, Funded Status and the Amounts Not Yet Recognized As Components of Net Periodic Pension and Postretirement Benefits Cost	Projected Benefit Obligation, Fair Value of Assets and Funded Status The projected benefit obligation, fair value of assets, funded status and the amounts not yet recognized as components of net periodic pension and postretirement benefits cost for the Retirement Plans and PBOP at December 31, 2016 and 2015, were as follows (in millions):   Retirement Plans   PBOP   For the Year Ended December 31,   For the Year Ended December 31,   2016   2015   2016   2015 Change in benefit obligation:               Benefit obligation at beginning of period $ 143.8   $ 149.9   $ 48.4   $ 55.1 Service cost 3.6   3.8   0.3   0.3 Interest cost 4.4   4.9   2.0   2.0 Plan participants’ contributions —   —   1.0   1.1 Actuarial loss (gain) 1.6   (1.9 )   (5.6 )   (5.6 ) Benefits paid (0.5 )   (0.5 )   (4.0 )   (4.5 ) Settlement (15.2 )   (12.4 )   —   — Benefit obligation at end of period $ 137.7   $ 143.8   $ 42.1   $ 48.4                 Change in plan assets:               Fair value of plan assets at beginning of period $ 119.5   $ 130.7   $ 86.4   $ 87.3 Actual return on plan assets 8.9   (1.3 )   2.3   2.4 Benefits paid (0.5 )   (0.5 )   (4.0 )   (4.5 ) Settlement (15.2 )   (12.4 )   —   — Company contributions 3.0   3.0   0.2   0.1 Plan participants’ contributions —   —   1.0   1.1 Fair value of plan assets at end of period $ 115.7   $ 119.5   $ 85.9   $ 86.4                 Funded status $ (22.0 )   $ (24.3 )   $ 43.8   $ 38.0                 Items not recognized as components of net periodic cost:             Prior service cost (credit) $ —   $ —   $ —   $ (0.9 ) Net actuarial loss 24.6   29.9   4.0   7.2 Total $ 24.6   $ 29.9   $ 4.0   $ 6.3
Schedule of Accumulated Benefit Obligations in Excess of Fair Value of Plan Assets	At December 31, 2016 and 2015, the following aggregate information relates only to the underfunded plans (in millions):   Retirement Plans   For the Year Ended December 31,   2016   2015 Projected benefit obligation $ 137.7   $ 143.8 Accumulated benefit obligation 128.2   134.1 Fair value of plan assets 115.7   119.5
Schedule of Net Benefit Costs	Components of Net Periodic Benefit Cost Components of net periodic benefit cost for both the Retirement Plans and PBOP for the years ended December 31, 2016, 2015 and 2014, were as follows (in millions):   Retirement Plans   PBOP   For the Year Ended December 31,   For the Year Ended December 31,   2016   2015   2014   2016   2015   2014 Service cost $ 3.6   $ 3.8   $ 3.9   $ 0.3   $ 0.3   $ 0.4 Interest cost 4.4   4.9   5.8   2.0   2.0   2.2 Expected return on plan assets (7.9 )   (9.1 )   (9.5 )   (4.6 )   (4.6 )   (4.2 ) Amortization of prior service credit —   —   —   (0.9 )   (7.7 )   (7.8 ) Amortization of unrecognized net loss 2.7   2.0   1.4   —   —   0.3 Settlement charge 3.2   2.5   1.9   —   —   — Net periodic benefit cost $ 6.0   $ 4.1   $ 3.5   $ (3.2 )   $ (10.0 )   $ (9.1 )
Schedule of Expected Benefit Payments	Estimated Future Benefit Payments The following table shows benefit payments, which reflect expected future service, as appropriate, which are expected to be paid for both the Retirement Plans and PBOP (in millions):   Retirement Plans   PBOP 2017 $ 18.4   $ 2.7 2018 12.6   2.8 2019 14.6   2.8 2020 14.5   2.9 2021 12.7   2.9 2022-2026 62.4   13.1
Weighted-Average Assumptions Used to Determine Benefit Obligations	Weighted–Average Assumptions Weighted-average assumptions used to determine benefit obligations for the years ended December 31, 2016 and 2015, were as follows:   Retirement Plans   PBOP   For the Year Ended December 31,   For the Year Ended December 31,   2016   2015   2016   2015   Pension   SRP   Pension   SRP         Discount rate 3.60 %   3.85 %   3.60 %   4.00 %   4.20 %   4.25 % Expected return on plan assets 7.25 %   7.25 %   7.50 %   7.50 %   5.30 %   5.30 % Rate of compensation increase 3.86 %   3.86 %   3.50 %   3.50 %   —   —
Weighted-Average Assumptions Used to Determine Net Periodic Benefit Cost	Weighted-average assumptions used to determine net periodic benefit cost for the periods indicated were as follows:   Retirement Plans   PBOP   For the Year Ended December 31,   For the Year Ended December 31,   2016   2015   2014   2016   2015   2014   Pension (1)   SRP   Pension (2)   SRP   Pension   SRP             Discount rate 3.60%/ 3.45%/ 3.00%/ 2.85%   4.00 %   3.35%/ 3.60%   3.75 %   4.00 %   4.25 %   4.25 %   3.90 %   4.50 % Expected return on plan assets 7.25 %   7.25 %   7.50%   7.50 %   7.50 %   7.50 %   5.30 %   5.30 %   5.30 % Rate of compensation increase 3.50 %   3.50 %   3.50%   3.50 %   3.50 %   3.50 %   —   —   — (1) Pension expense was remeasured quarterly in 2016 to reflect settlements. (2) Pension expense was remeasured at August 31, 2015, to reflect a settlement.
Schedule of Effect of One-Percentage-Point Change in Assumed Health Care Cost Trend Rates	PBOP Assumed Health Care Cost Trends Assumed health care cost trend rates have a significant effect on the amounts reported for PBOP. A one-percentage-point change in assumed trend rates for health care costs would have had the following effects on amounts reported for the year ended December 31, 2016 (in millions): Effect of 1% Increase:   2016 Benefit obligation at end of year   $ 1.7 Total of service and interest costs for year   0.1 Effect of 1% Decrease:     Benefit obligation at end of year   $ (1.5 ) Total of service and interest costs for year   (0.1 )
Fair Values of Pension Plan Assets By Asset Class Master Trust	The following table sets forth, by level within the fair value hierarchy, a summary of the Master Trust’s investments measured at fair value on a recurring basis at December 31, 2016 (in millions):   Master Trust Assets   Measured under Fair Value Hierarchy   Measured at Net Asset Value   Total Master Trust Assets   Level 1   Level 2   Level 3   Total Equity securities $ 40.6   $ —   $ —   $ 40.6   $ —   $ 40.6 Short-term investments 6.7   —   —   6.7   —   6.7 Fixed income mutual funds 93.1   —   —   93.1   —   93.1 Asset-backed securities —   7.1   —   7.1   —   7.1 Total assets measured at fair    value 140.4 7.1 — 147.5   —   147.5 Total limited partnerships measured at net asset value (1) —   —   —   —   82.5   82.5 Total $ 140.4   $ 7.1   $ —   $ 147.5   $ 82.5   $ 230.0 The following table sets forth, by level within the fair value hierarchy, a summary of the Master Trust’s investments measured at fair value on a recurring basis at December 31, 2015 (in millions):   Master Trust Assets   Measured under Fair Value Hierarchy   Measured at Net Asset Value   Total Master Trust Assets   Level 1   Level 2   Level 3   Total Equity securities $ 36.1   $ —   $ —   $ 36.1   $ —   $ 36.1 Short-term investments 6.6   —   —   6.6   —   6.6 Fixed income mutual funds 94.8   —   —   94.8   —   94.8 Asset-backed securities —   6.4   —   6.4   —   6.4 Total assets measured at fair value 137.5   6.4   —   143.9   —   143.9 Total limited partnerships measured at net asset value (1) —   —   —   —   90.4   90.4 Total $ 137.5 $ 6.4 $ — $ 143.9 $ 90.4 $ 234.3 (1) In May 2015, the FASB issued Accounting Standards Update No. 2015-07, “Disclosures for Investments in Certain Entities that Calculate Net Asset Value per Share (or its Equivalent)” (ASU 2015-07), which removes the requirement to present certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient within the fair value hierarchy table. The fair value amounts presented in the tables above are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the statement of financial position. The Partnership adopted ASU 2015-07 as of December 31, 2016, and has retrospectively applied ASU 2015-07 as required. Other than the presentation of the investments measured at net asset value, there were no effects to the reported amounts presented for the Master Trust as of December 31, 2015.
Fair Values of Pension Plan Assets By Asset Class	The following table sets forth, by level within the fair value hierarchy, a summary of the PBOP trust investments measured at fair value on a recurring basis at December 31, 2016 (in millions):   PBOP Trust Assets   Level 1   Level 2   Level 3   Total Short-term investments $ 3.2   $ —   $ —   $ 3.2 Fixed income mutual funds 4.9   —   —   4.9 Asset-backed securities —   15.5   —   15.5 Corporate bonds —   18.6   —   18.6 Tax exempt securities —   43.7   —   43.7 Total investments $ 8.1   $ 77.8   $ —   $ 85.9 The following table sets forth, by level within the fair value hierarchy, a summary of the PBOP trust investments measured at fair value on a recurring basis at December 31, 2015 (in millions):   PBOP Trust Assets   Level 1   Level 2   Level 3   Total Short-term investments $ 3.0   $ —   $ —   $ 3.0 Fixed income mutual funds 4.7   —   —   4.7 Asset-backed securities —   18.8   —   18.8 Corporate bonds —   17.0   —   17.0 Tax exempt securities —   42.9   —   42.9 Total investments $ 7.7   $ 78.7   $ —   $ 86.4
Disclosure of Share Based Compensation Arrangements by Share Based Payment Award LTIP	A summary of the status of the Phantom Common Units granted under the Partnership’s LTIP as of December 31, 2016 and 2015, and changes during the years ended December 31, 2016 and 2015, is presented below:   Phantom Common Units   Total Fair Value (in millions)   Weighted-Average Vesting Period  (in years) Outstanding at January 1, 2015 (1) 199,669   $ 4.1   0.9 Granted 647,256   10.1   2.4 Paid (196,748 )   (2.9 )   — Forfeited (4,209 )   —   — Outstanding at December 31, 2015 (1) 645,968   8.7   1.5 Granted 865,091   10.2   2.3 Paid (237,972 )   (4.1 )   — Forfeited (15,462 )   —   — Outstanding at December 31, 2016 (1) 1,257,625   $ 22.5   1.2 (1) Represents fair value and remaining weighted-average vesting period of outstanding awards at the end of the period.