Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes		$ 219.6	$ 190.1
Income tax benefit		(53.1)	(48.4)
Net income		166.5	141.7
Unfunded Employee Benefit Obligations | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Amortization of prior service costs	[1]	(0.8)	(1.0)
Amortization of actuarial losses	[1]	(2.5)	(2.6)
Income before taxes		(3.3)	(3.6)
Income tax benefit		0.8	0.9
Net income		$ (2.5)	$ (2.7)

[1] These AOCI components are included in the computation of net pension and postretirement benefit costs. See Note 13, Employee Benefit Plans and Other Postretirement Benefits, for additional information.