Stockholders' Equity - Changes in Accumulated Other Comprehensive Income by Component (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at December 31, 2013	$ (65.0)
Other comprehensive income (loss) before reclassifications, net of tax	(96.6)
Amounts reclassified from AOCI, net of tax	7.7
Other comprehensive income (loss)	(88.9)	49.0	(16.2)
Balance at December 31, 2014	(153.9)	(65.0)
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at December 31, 2013	(0.1)
Other comprehensive income (loss) before reclassifications, net of tax	(2.6)
Amounts reclassified from AOCI, net of tax	0
Other comprehensive income (loss)	(2.6)
Balance at December 31, 2014	(2.7)
Unfunded Employee Benefit Obligations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at December 31, 2013	(36.3)
Other comprehensive income (loss) before reclassifications, net of tax	(94.0)
Amounts reclassified from AOCI, net of tax	4.2
Other comprehensive income (loss)	(89.8)
Balance at December 31, 2014	(126.1)
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, 2013	(28.2)
Other comprehensive income (loss) before reclassifications, net of tax	0
Amounts reclassified from AOCI, net of tax	3.5
Other comprehensive income (loss)	3.5
Balance at December 31, 2014	(24.7)
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, 2013	(0.4)
Other comprehensive income (loss) before reclassifications, net of tax	0
Amounts reclassified from AOCI, net of tax	0
Other comprehensive income (loss)	0
Balance at December 31, 2014	(0.4)
Scenario, Forecast
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Net amount of settlement gains (losses) on derivative instruments included in accumulated OCI to be amortized over next 12 months, before tax				(5.7)
Net amount of settlement gains (losses) on derivative instruments included in accumulated OCI to be amortized over next 12 months, after tax				$ (3.4)