Other Postretirement Benefit Plans - Change in Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets:
Company contributions / payments	$ 17.9	$ 35.4	$ 11.3
Accrued postretirement liability
Change in plan assets:
Fair value of plan assets at beginning of year	64.4	72.3
Company contributions / payments	2.7	8.0
Transfer to VEBA trust for certain active employees' medical benefits	(50.0)	0.0
Return on plan assets	0.8	6.8
Benefits paid	(6.8)	(22.7)
Fair value of plan assets at end of year	11.1	64.4	$ 72.3
Funded status at end of year	$ (46.5)	$ 1.0