Pension Plans and Other Postretirement Benefits - Pension and Postretirement Plan Contributions (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Retirement Benefits [Abstract]
Employer contributions	$ 4.2	$ 2.1	$ 18.9	$ 5.2
Payment for other postretirement benefits	$ 0.8	$ 0.8	$ 1.4	$ 1.5