Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes		$ 318.6	$ 244.5	$ 772.8	$ 765.6
Income tax expense		(80.5)	(61.3)	(188.8)	(189.6)
Net income		238.1	183.2	584.0	576.0
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)	(3.8)	(3.6)
Amortization of actuarial losses	[1]	(0.1)	(0.9)	(0.3)	(2.8)
Income before taxes		(1.4)	(2.1)	(4.1)	(6.4)
Income tax expense		0.4	0.5	1.0	1.6
Net income		$ (1.0)	$ (1.6)	$ (3.1)	$ (4.8)

[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.