Adoption of Revenue Recognition - Effect of Revenue Recognition on Condensed Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 14.0	$ 5.9
Other comprehensive income:
Amortization of actuarial losses and prior service credits, net of income taxes	12.5	9.7
Comprehensive income	26.5	$ 15.6
Effect of Adoption | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	(5.9)
Other comprehensive income:
Amortization of actuarial losses and prior service credits, net of income taxes	0.0
Comprehensive income	(5.9)
Amounts Excluding Effect of Adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	8.1
Other comprehensive income:
Amortization of actuarial losses and prior service credits, net of income taxes	12.5
Comprehensive income	$ 20.6