Pension and Postretirement Plans - Amounts Recognized in Balance Sheet (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Pension and postretirement benefit obligations (noncurrent liability)	$ 177.5		$ 230.6
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Capitalized portion of defined benefit contribution percent	13.00%	13.00%
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Accrued payroll and benefits (current liability)	$ 2.1		2.1
Pension and postretirement benefit obligations (noncurrent liability)	95.0		122.3
Total	97.1		124.4
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Prior service (cost) benefit, net of tax of ($0.1), ($0.1), $4.6, $5.3	(0.1)		(0.1)
Prior service costs, tax	(0.1)		(0.1)
Actuarial loss, net of tax of ($45.0), ($42.5), ($7.1), ($12.7)	(148.4)		(156.3)
Actuarial loss, tax	(42.5)		(45.0)
Total	(148.5)		(156.4)
Other Postretirement Benefits Plan
Defined Benefit Plan Disclosure [Line Items]
Accrued payroll and benefits (current liability)	10.5		11.0
Pension and postretirement benefit obligations (noncurrent liability)	80.6		106.9
Total	91.1		117.9
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Prior service (cost) benefit, net of tax of ($0.1), ($0.1), $4.6, $5.3	19.9		17.5
Prior service costs, tax	5.3		4.6
Actuarial loss, net of tax of ($45.0), ($42.5), ($7.1), ($12.7)	(44.5)		(25.6)
Actuarial loss, tax	(12.7)		(7.1)
Total	$ (24.6)		(8.1)
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year			11.0
Nonqualified Plan | Pension Plan
Accumulated Other Comprehensive Income (Loss), Net of Tax [Abstract]
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year			$ 3.0