Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2024	Mar. 31, 2023
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes		$ 187.5	$ 253.3
Income tax benefit		(40.6)	(63.2)
Net income		146.9	190.1
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]	(1.3)	(1.2)
Amortization of actuarial losses	[1]	(0.1)	(1.0)
Income before taxes		(1.4)	(2.2)
Income tax benefit		0.4	0.6
Net income		$ (1.0)	$ (1.6)

[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.