Pension and Other Postretirement Benefits (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) plan	Dec. 31, 2019 USD ($) plan
Underfunded
Pension and Other Postretirement Benefits
Actuarial loss due to change in discount rates	$ 22.9	$ 33.6
Actuarial loss due to change in lump sum interest rates	14.0	15.9
Actuarial gain attributable to reduction in salary scale	3.3
Gain (loss) due to plan experience different than anticipated		3.4
Overfunded
Pension and Other Postretirement Benefits
Actuarial loss due to change in discount rates	3.6	3.7
Actuarial loss due to change in lump sum interest rates	$ 1.7
Actuarial gain due to decrease in inflation assumption		$ 0.8
Number of defined benefit plans | plan	1	1
Postemployment Retirement Benefits
Pension and Other Postretirement Benefits
Actuarial loss due to change in discount rates	$ 1.4	$ 2.2
Gain (loss) due to plan experience different than anticipated	$ (1.0)
Minimum
Pension and Other Postretirement Benefits
Age limit	50 years
Maximum
Pension and Other Postretirement Benefits
Age limit	65 years