Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes		$ 267.8	$ 400.6	$ 521.1	$ 741.0
Income tax benefit		(65.1)	(99.1)	(128.3)	(185.3)
Net income		202.7	301.5	392.8	555.7
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)	(2.4)	(1.6)
Amortization of actuarial losses	[1]	(1.0)	(0.7)	(1.9)	(1.4)
Income before taxes		(2.2)	(1.5)	(4.3)	(3.0)
Income tax benefit		0.6	0.4	1.1	0.7
Net income		$ (1.6)	$ (1.1)	$ (3.2)	$ (2.3)

[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.