Employee Benefits - Change in Benefit Obligations and Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Mar. 31, 2015
Employee Benefit Plans
New 401k Contribution for new hires				6.00%
Amounts recognized on the entity's balance sheets related to the funded status of the benefit plans
Pension and OPEB obligations	$ 167.6	$ 210.9
Pension Benefits
Change in benefit obligation
Obligation at January 1	1,290.6	1,315.2
Service cost	10.5	14.7	$ 9.4
Interest cost	49.7	52.9	59.3
Participant contributions	0.0	0.0
Plan amendments	(2.6)	0.0
Transfer to affiliates	(121.1)	(2.4)
Actuarial loss (gain), net	25.3	(11.5)
Benefit payments	(75.4)	(78.3)
Obligation at December 31	1,177.0	1,290.6	1,315.2
Change in fair value of plan assets
Beginning balance at January 1	1,179.3	1,160.0
Actual return on plan assets	73.0	(7.8)
Employer contributions	5.3	105.0
Participant contributions	0.0	0.0
Transfer to affiliates	(79.4)	0.4
Benefit payments	(75.4)	(78.3)
Ending balance at December 31	1,102.8	1,179.3	1,160.0
Funded status at December 31	(74.2)	(111.3)
Amounts recognized on the entity's balance sheets related to the funded status of the benefit plans
Other long-term assets	0.0	0.0
Pension and OPEB obligations	74.2	111.3
Total net liabilities	(74.2)	(111.3)
Accumulated benefit obligation	1,175.8	1,287.5
Information for pension plans with an accumulated benefit obligation in excess of plan assets
Projected benefit obligation	1,177.0	1,290.2
Accumulated benefit obligation	1,175.8	1,289.5
Fair value of plan assets	1,102.8	1,178.9
OPEB
Change in benefit obligation
Obligation at January 1	313.8	322.3
Service cost	7.3	9.0	8.1
Interest cost	13.2	13.4	14.4
Participant contributions	8.8	8.8
Plan amendments	0.0	0.0
Transfer to affiliates	(17.0)	0.0
Actuarial loss (gain), net	(9.7)	(22.3)
Benefit payments	(19.0)	(18.7)
Federal subsidy on benefits paid	1.1	1.3
Obligation at December 31	298.5	313.8	322.3
Change in fair value of plan assets
Beginning balance at January 1	216.1	224.9
Actual return on plan assets	13.5	(1.5)
Employer contributions	2.7	2.6
Participant contributions	8.8	8.8
Transfer to affiliates	(17.0)	0.0
Benefit payments	(19.0)	(18.7)
Ending balance at December 31	205.1	216.1	$ 224.9
Funded status at December 31	(93.4)	(97.7)
Amounts recognized on the entity's balance sheets related to the funded status of the benefit plans
Other long-term assets	0.0	1.9
Pension and OPEB obligations	93.4	99.6
Total net liabilities	$ (93.4)	$ (97.7)