Adoption of Revenue Recognition - Effect of Revenue Recognition on Condensed Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 34.8	$ 24.3	$ 48.8	$ 30.2
Other comprehensive income:
Amortization of actuarial losses and prior service credits, net of income taxes	12.5	9.4	25.0	19.1
Comprehensive income	47.3	$ 33.7	73.8	$ 49.3
Effect of Adoption | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	(2.4)		(8.3)
Other comprehensive income:
Amortization of actuarial losses and prior service credits, net of income taxes	0.0		0.0
Comprehensive income	(2.4)		(8.3)
Amounts Excluding Effect of Adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	32.4		40.5
Other comprehensive income:
Amortization of actuarial losses and prior service credits, net of income taxes	12.5		25.0
Comprehensive income	$ 44.9		$ 65.5