Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2022	Jun. 30, 2021	Jun. 30, 2022	Jun. 30, 2021
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes		$ 400.6	$ 274.7	$ 741.0	$ 494.3
Income tax benefit		(99.1)	(67.4)	(185.3)	(120.5)
Net income		301.5	207.3	555.7	373.8
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]	(0.8)	(0.8)	(1.6)	1.7
Amortization of actuarial losses	[1]	(0.7)	(2.5)	(1.4)	(5.0)
Income before taxes		(1.5)	(3.3)	(3.0)	(6.7)
Income tax benefit		(0.4)	(0.8)	(0.7)	1.7
Net income		$ (1.1)	$ (2.5)	$ (2.3)	$ (5.0)

[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.