Comprehensive Income - Reclassifications from Accumulated Other Comprehensive Loss Amortization of Pension Plan Cost (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 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	[1]	$ 0.5	$ 0.7	$ 0.9	$ 1.3
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		0.5	0.7	0.9	1.3
Income tax expense		0.2	0.2	0.3	0.3
Reclassifications, net of tax		$ 0.3	$ 0.5	$ 0.6	$ 1.0

[1] (a) This accumulated other comprehensive loss component is included in the calculation of net periodic pension and other postretirement benefits plan income recognized in investment and other income in the unaudited condensed consolidated statements of operations (see Note 9, Retirement Plans).