Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes		$ 266.7	$ 267.8	$ 454.3	$ 521.1
Income tax benefit		67.8	65.1	108.4	128.3
Net income		198.9	202.7	345.9	392.8
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)	(2.6)	(2.4)
Amortization of actuarial losses	[1]	(0.1)	(1.0)	(0.2)	(1.9)
Income before taxes		(1.4)	(2.2)	(2.8)	(4.3)
Income tax benefit		0.3	0.6	0.7	1.1
Net income		$ (1.1)	$ (1.6)	$ (2.1)	$ (3.2)

[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.