Stockholders' Equity - Reclassifications Out of Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest expense, net	$ (25.2)	$ (22.5)	$ (49.2)	$ (44.1)
Income tax benefit	(65.4)	(61.8)	(127.1)	(117.3)
Net income	143.2	115.9	260.6	219.6
Income before taxes	208.6	177.7	387.7	336.9
Unfunded Employee Benefit Obligations | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax benefit	1.2	1.0	2.3	2.0
Net income	(2.1)	(1.6)	(4.3)	(3.2)
Amortization of prior service costs	(1.4)	(1.4)	(2.9)	(2.8)
Amortization of actuarial losses	(1.9)	(1.2)	(3.7)	(2.4)
Income before taxes	(3.3)	(2.6)	(6.6)	(5.2)
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	(1.4)	(1.4)	(2.8)	(2.8)
Income tax benefit	0.6	0.6	1.2	1.1
Net income	$ (0.8)	$ (0.8)	$ (1.6)	$ (1.7)