Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2023	Mar. 31, 2022
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes		$ 253.3	$ 340.3
Income tax benefit		(63.2)	(86.1)
Net income		190.1	254.2
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]	(1.2)	(0.8)
Amortization of actuarial losses	[1]	(1.0)	(0.7)
Income before taxes		(2.2)	(1.5)
Income tax benefit		0.6	0.3
Net income		$ (1.6)	$ (1.2)

[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.