Stockholders' Equity - Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at December 31, 2017	$ 2,182.6
Other comprehensive income before reclassifications	2.4
Amounts reclassified from AOCI	15.7
Other comprehensive income (loss)	18.1	$ (17.3)	$ (14.7)
Balance at December 31, 2018	2,672.4	2,182.6
Balance at December 31, 2017	2,182.6
Balance at December 31, 2018	2,672.4	2,182.6
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at December 31, 2017	(0.3)
Amounts reclassified from AOCI	(0.1)
Other comprehensive income (loss)	(0.1)
Balance at December 31, 2018	(0.4)	(0.3)
Balance at December 31, 2017	(0.3)
Balance at December 31, 2018	(0.4)	(0.3)
Unfunded Employee Benefit Obligations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at December 31, 2017	(142.1)
Other comprehensive income before reclassifications	2.4
Amounts reclassified from AOCI	11.8
Other comprehensive income (loss)	14.2
Balance at December 31, 2018	(127.9)	(142.1)
Balance at December 31, 2017	(142.1)
Balance at December 31, 2018	(127.9)	(142.1)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at December 31, 2017	(156.9)
Balance at December 31, 2018	(138.8)	(156.9)
Balance at December 31, 2017	(156.9)
Balance at December 31, 2018	(138.8)	(156.9)
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 December 31, 2017	(14.2)
Amounts reclassified from AOCI	4.0
Other comprehensive income (loss)	4.0
Balance at December 31, 2018	(10.2)	(14.2)
Balance at December 31, 2017	(14.2)
Balance at December 31, 2018	(10.2)	(14.2)
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 December 31, 2017	(0.3)
Balance at December 31, 2018	(0.3)	(0.3)
Balance at December 31, 2017	(0.3)
Balance at December 31, 2018	$ (0.3)	$ (0.3)