Retirement Plans - Amounts Recognized in Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan
Amortization of:
Net actuarial loss	$ 3.1	$ 1.8	$ 2.5
Amounts arising during the period:
Settlements	0.0	3.9	0.0
Net actuarial gain/(loss)	1.7	(11.8)	(5.6)
Total	4.8	(6.1)	(3.1)
Other Postretirement Benefit Plan
Amortization of:
Net actuarial loss	0.0	0.0	0.0
Amounts arising during the period:
Settlements	0.0	0.0	0.0
Net actuarial gain/(loss)	(0.2)	(0.6)	0.0
Total	$ (0.2)	$ (0.6)	$ 0.0