Stockholders' Equity - Changes in Accumulated Other Comprehensive Income by Component (Details) $ in Millions	6 Months Ended
	Jun. 30, 2020 USD ($)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at January 1, 2020	$ 3,071.0
Other comprehensive income before reclassifications, net of tax	0.7
Amounts reclassified from AOCI, net of tax	5.4
Balance at June 30, 2020	3,133.1
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at January 1, 2020	(0.4)
Balance at June 30, 2020	(0.4)
Unfunded Employee Benefit Obligations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at January 1, 2020	(158.9)
Amounts reclassified from AOCI, net of tax	5.4
Balance at June 30, 2020	(153.5)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Balance at January 1, 2020	(159.5)
Balance at June 30, 2020	(153.4)
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, 2020	(0.2)
Balance at June 30, 2020	(0.2)
Marketable Debt Securities | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Other comprehensive income before reclassifications, net of tax	0.7
Balance at June 30, 2020	$ 0.7