Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment Out Of Accumulated Other Comprehensive Income [Line Items]
Interest expense, net		$ (22.4)	$ (23.8)	$ (46.4)	$ (49.6)
Income before taxes		256.0	245.3	505.3	431.8
Income tax benefit		(62.4)	(58.7)	(124.9)	(105.1)
Net income		193.6	186.6	380.4	326.7
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]	(1.5)	(1.7)	(3.0)	(3.3)
Amortization of actuarial gains / (losses)	[1]	(1.6)	(2.3)	(3.3)	(4.6)
Income before taxes		(3.1)	(4.0)	(6.3)	(7.9)
Income tax benefit		0.8	1.0	1.6	1.9
Net income		(2.3)	(3.0)	(4.7)	(6.0)
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)	(1.3)	(2.6)	(2.7)
Income tax benefit		0.3	0.3	0.7	0.7
Net income		$ (1.0)	$ (1.0)	$ (1.9)	$ (2.0)

[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.

[2] 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 net amount of settlement gains or losses on derivative instruments included in AOCI to be amortized over the next 12 months is a net loss of $5.2 million ($3.9 million after tax). For a discussion of treasury lock derivative instrument activity, see Note 14, Derivative Instruments and Hedging Activities, of the Notes to Consolidated Financial Statements in "Part II, Item 8. Financial Statements and Supplementary Data" of our 2018 Annual Report on Form 10-K