Summary of Significant Accounting Policies - Interest and Other Income (Expense), Net (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Accounting Policies [Abstract]
Interest income	$ 0.2	$ 1.8	$ 0.8	$ 2.0
Pension and postretirement benefit (expense)	(0.6)	(1.1)	(1.3)	(2.3)
Gain (loss) on sale of joint venture interest	0.0	0.0	0.0	2.6
Other income (expense), net	0.0	(0.1)	0.3	(0.1)
Interest and other income (expense), net	$ (0.4)	$ 0.6	$ (0.2)	$ 2.2