Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Interest expense, net		$ (19.6)	$ (24.1)
Income before taxes		190.1	249.3
Income tax benefit		(48.4)	(62.5)
Net income		141.7	186.8
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]	(1.0)	(1.5)
Amortization of actuarial losses	[1]	(2.6)	(1.6)
Income before taxes		(3.6)	(3.1)
Income tax benefit		0.9	0.8
Net income		$ (2.7)	(2.3)
Treasury Lock | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Interest expense, net	[2]		(1.3)
Income tax benefit			0.3
Net income			$ (1.0)

[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.

[2] For 2019, this AOCI component is included in interest expense, net. Amount relates to the amortization of the effective portion of treasury lock derivative instruments recorded in AOCI. The remaining balances of the treasury locks were written off as a result of the Company’s November 2019 debt refinancing. For a discussion of treasury lock derivative instrument activity, see Note 16, Derivative Instruments and Hedging Activities, of the Notes to Consolidated Financial Statements in "Part II, Item 8. Financial Statements and Supplementary Data" of our 2019 Annual Report on Form 10-K