Postretirement Benefits Other than Pensions - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Unrecognized actuarial losses	$ 3.1
Unrecognized actuarial losses, net of tax	1.7
Unrecognized prior service credit	11.1
Unrecognized prior service credit, net of tax	6.1
Postretirement Health Coverage [Member]
Defined Benefit Plan Disclosure [Line Items]
Net gain (loss) recognized in other comprehensive income	0.1	$ (0.3)
Amortization of net loss	0.3	0.2	$ 0.2
Amortization of prior service credit recognized in other comprehensive income	$ 1.3	1.3	$ 1.3
Unrecognized actuarial losses		3.5
Unrecognized actuarial losses, net of tax		1.9
Unrecognized prior service credit		12.4
Unrecognized prior service credit, net of tax		$ 6.9