Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Income before taxes		$ 1,014.1	$ 1,364.8	$ 1,108.7
Income tax benefit		(248.9)	(335.0)	(267.6)
Net income		765.2	1,029.8	$ 841.1
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]	(4.8)	(3.2)
Amortization of actuarial gains / (losses)	[1]	(3.7)	(3.0)
Income before taxes		(8.5)	(6.2)
Income tax benefit		2.1	1.5
Net income		$ (6.4)	$ (4.7)

[1] These AOCI components are included in the computation of net pension and postretirement benefit costs. See Note 12, Employee Benefit Plans and Other Postretirement Benefits, for additional information.