Postretirement Benefits Other than Pensions - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Unrecognized actuarial losses	$ 3.0	$ 3.1
Unrecognized actuarial losses, net of tax	2.0	2.2
Unrecognized prior service credit	9.2	9.8
Unrecognized prior service credit, net of tax	6.4	7.0
Postretirement Health Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss) recognized in other comprehensive income	0.1	(0.2)	$ 0.1
Amortization of net loss	0.2	0.2	0.3
Amortization of prior service credit recognized in other comprehensive income	$ 0.6	$ 1.3	$ 1.3