Comprehensive Income - Reclassifications from Accumulated Other Comprehensive Loss Amortization of Pension Plan Cost (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Accumulated Defined Benefit Plans Adjustment, Net Actuarial loss
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of pension and other postretirement benefits plan cost: Reclassifications before tax	$ 0.4	$ 0.6	$ 1.3	$ 1.9
Pension and Other Postretirement Benefits Plan Cost
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of pension and other postretirement benefits plan cost: Reclassifications before tax	0.4	0.6	1.3	1.9
Income tax expense	0.1	0.2	0.4	0.5
Reclassifications, net of tax	$ 0.3	$ 0.4	$ 0.9	$ 1.4