Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes		$ 347.4	$ 333.9	$ 1,088.3	$ 828.3
Income tax benefit		(84.9)	(83.2)	(270.1)	(203.7)
Net income		262.5	250.7	818.2	624.6
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]	(0.8)	(0.8)	(2.4)	(2.5)
Amortization of actuarial losses	[1]	(0.8)	(2.5)	(2.3)	(7.5)
Income before taxes		(1.6)	(3.3)	(4.7)	(10.0)
Income tax benefit		0.4	0.8	1.2	2.5
Net income		$ (1.2)	$ (2.5)	$ (3.5)	$ (7.5)

[1] These AOCI components are included in the computation of net pension and postretirement benefit costs. See Note 14, Employee Benefit Plans and Other Postretirement Benefits, for additional information.