Adoption of Revenue Recognition Guidance - Effect of Revenue Recognition on Condensed Consolidated Statement of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income	$ 35.4	$ 31.7	$ 39.2	$ 31.4	$ 25.3	$ 32.9	$ 44.1	$ 38.7	$ 137.7	$ 141.0	$ 137.3
Other comprehensive income:
Actuarial losses and amortization of actuarial losses, net of income taxes											33.2
Comprehensive income									$ 146.6	$ 143.9	170.5
Effect of Adoption | ASU 2014-09
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income											(9.6)
Other comprehensive income:
Actuarial losses and amortization of actuarial losses, net of income taxes											0.0
Comprehensive income											(9.6)
Amounts Excluding Effect of Adoption
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Net income											127.7
Other comprehensive income:
Actuarial losses and amortization of actuarial losses, net of income taxes											33.2
Comprehensive income											$ 160.9