Benefit Plans and Other Postretirement Benefits, OPEB benefit obligation and benefit expense (Details) - Other Postretirement Benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Postretirement Benefits [Line Items]
Projected benefit obligation	$ 6.8	$ 7.9
Benefit obligation discount rate (as a percent)	2.40%	3.10%
Components of net postretirement benefit expense
Net postretirement benefit expense	$ 0.2	$ 0.3	$ 0.8