Stockholders' Equity - Changes in Accumulated Other Comprehensive Income by Component (Details) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at January 1, 2019	$ 2,672.4
Amounts reclassified from AOCI, net of tax	3.3
Balance at March 31, 2019	2,794.6
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at January 1, 2019	(0.4)
Balance at March 31, 2019	(0.4)
Unfunded Employee Benefit Obligations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at January 1, 2019	(127.9)
Amounts reclassified from AOCI, net of tax	2.3
Balance at March 31, 2019	(125.6)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at January 1, 2019	(138.8)
Amounts reclassified from AOCI, net of tax	3.3
Balance at March 31, 2019	(135.5)
Treasury Lock | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at January 1, 2019	(10.2)
Amounts reclassified from AOCI, net of tax	1.0
Balance at March 31, 2019	(9.2)
Foreign Exchange Contract | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at January 1, 2019	(0.3)
Balance at March 31, 2019	$ (0.3)