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
Defined 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
Long-term liabilities	$ 498.6	$ 543.1
Pension Benefits
Change in benefit obligation
Obligation at January 1	3,083.0	1,505.5
Obligation assumed from acquisition	0.0	1,594.0
Service cost	45.4	30.4	$ 10.1
Interest cost	130.8	94.3	68.1
Participant contributions	0.0	0.0
Plan amendments	(3.0)	0.0
Actuarial loss (gain)	71.7	14.6
Benefit payments	(269.1)	(156.0)
Plan curtailments	0.0	0.2
Obligation at December 31	3,058.8	3,083.0	1,505.5
Change in fair value of plan assets
Beginning balance at January 1	2,755.1	1,444.6
Assets received from acquisition	0.0	1,420.9
Actual return on plan assets	199.4	(62.1)
Employer contributions	23.8	107.7
Participant contributions	0.0	0.0
Benefit payments	(269.1)	(156.0)
Ending balance at December 31	2,709.2	2,755.1	1,444.6
Funded status at December 31	(349.6)	(327.9)
Amounts recognized on the entity's balance sheets related to the funded status of the benefit plans
Long-term assets	74.4	74.1
Long-term liabilities	424.0	402.0
Total net liabilities	(349.6)	(327.9)
Benefit obligations held for sale		0.8
Accumulated benefit obligation	2,939.9	2,936.4
Information for pension plans with an accumulated benefit obligation in excess of plan assets
Projected benefit obligation	1,667.0	1,706.6
Accumulated benefit obligation	1,549.5	1,560.5
Fair value of plan assets	1,242.9	1,304.6
OPEB
Change in benefit obligation
Obligation at January 1	842.0	397.7
Obligation assumed from acquisition	0.0	493.0
Service cost	26.1	20.7	8.5
Interest cost	37.0	26.7	17.8
Participant contributions	16.4	12.7
Plan amendments	(18.9)	0.0
Actuarial loss (gain)	(36.5)	(74.0)
Benefit payments	(49.1)	(36.2)
Federal subsidy on benefits paid	1.4	1.6
Plan curtailments	0.0	(0.2)
Obligation at December 31	818.4	842.0	397.7
Change in fair value of plan assets
Beginning balance at January 1	749.8	333.5
Assets received from acquisition	0.0	442.1
Actual return on plan assets	51.5	(15.6)
Employer contributions	4.9	13.3
Participant contributions	16.4	12.7
Benefit payments	(49.1)	(36.2)
Ending balance at December 31	773.5	749.8	$ 333.5
Funded status at December 31	(44.9)	(92.2)
Amounts recognized on the entity's balance sheets related to the funded status of the benefit plans
Long-term assets	29.7	50.1
Long-term liabilities	74.6	142.3
Total net liabilities	$ (44.9)	(92.2)
Benefit obligations held for sale		$ 0.4