Employee Benefit Plans and Other Postretirement Benefits - Change in Benefit Obligations and Change in Fair Value of Plan Assets Related to Pension and Postretirement Plans (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value at beginning of period		$ 830.4
Fair value of plan assets at plan year end		954.8	$ 830.4
Amounts Recognized on Consolidated Balance Sheets
Noncurrent liabilities		(372.5)	(357.2)
Pension Plans
Change in Benefit Obligations [Roll Forward]
Benefit obligation at beginning of period		1,158.4	1,092.5
Service cost		23.7	24.5	$ 24.0
Interest cost		41.6	40.9	46.2
Plan amendments		17.3	1.8
Actuarial loss (gain)	[1]	99.3	35.3
Participant contributions			0.0
Benefits paid		(40.1)	(36.6)
Benefit obligation at plan year end		1,300.2	1,158.4	1,092.5
Accumulated benefit obligation portion of above		1,237.7	1,116.6
Change in Fair Value of Plan Assets [Roll Forward]
Plan assets at fair value at beginning of period		830.4	764.4
Actual return on plan assets		121.0	44.4
Company contributions		43.5	58.2
Participant contributions			0.0
Benefits paid		(40.1)	(36.6)
Fair value of plan assets at plan year end		954.8	830.4	764.4
Underfunded status		(345.4)	(328.0)
Amounts Recognized on Consolidated Balance Sheets
Current liabilities		(1.4)	(1.3)
Noncurrent liabilities		(344.0)	(326.7)
Accrued obligation recognized at December 31		(345.4)	(328.0)
Amounts Recognized in Accumulated Other Comprehensive Loss (Income) (Pre-Tax)
Prior service cost (credit)		32.7	21.1
Actuarial loss (gain)		208.5	183.9
Total		241.2	205.0
Postretirement Plans
Change in Benefit Obligations [Roll Forward]
Benefit obligation at beginning of period		19.5	21.4
Service cost		0.3	0.5	1.7
Interest cost		0.6	0.6	1.2
Plan amendments		(0.5)	(5.3)
Actuarial loss (gain)	[1]	(2.2)	3.7
Participant contributions		1.3	1.3
Benefits paid		(2.5)	(2.7)
Benefit obligation at plan year end		16.5	19.5	$ 21.4
Change in Fair Value of Plan Assets [Roll Forward]
Company contributions		1.2	1.4
Participant contributions		1.3	1.3
Benefits paid		(2.5)	(2.7)
Underfunded status		(16.5)	(19.5)
Amounts Recognized on Consolidated Balance Sheets
Current liabilities		(1.0)	(1.2)
Noncurrent liabilities		(15.5)	(18.3)
Accrued obligation recognized at December 31		(16.5)	(19.5)
Amounts Recognized in Accumulated Other Comprehensive Loss (Income) (Pre-Tax)
Prior service cost (credit)		(5.3)	(5.0)
Actuarial loss (gain)		(4.0)	(1.9)
Total		$ (9.3)	$ (6.9)

[1] The actuarial loss in 2017 was due primarily to a decrease in the weighted average discount rate used to estimate our pension benefit obligations. In 2016, a decrease in the weighted average discount rate used to estimate our pension benefit obligations and changes in mortality assumptions from the Society of Actuaries resulted in an actuarial loss.