Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2022	Mar. 31, 2021
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes		$ 340.3	$ 219.6
Income tax benefit		(86.1)	(53.1)
Net income		254.2	166.5
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)	(0.8)
Amortization of actuarial losses		0.7	(2.5)	[1]
Income before taxes		(1.5)	(3.3)
Income tax benefit		(0.3)	(0.8)
Net income			$ (2.5)

[1] These AOCI components are included in the computation of net pension and postretirement benefit costs. See Note 14, Employee Benefit Plans and Other Postretirement Benefits, for additional information.