Stockholders' Equity - Changes in Accumulated Other Comprehensive Income by Component (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 3,246.3
Ending balance	3,608.3
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(0.4)
Other comprehensive income before reclassifications, net of tax	0.0
Amounts reclassified from AOCI, net of tax	0.4
Ending balance	(0.0)
Unfunded Employee Benefit Obligations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(144.2)
Other comprehensive income before reclassifications, net of tax	0.0
Amounts reclassified from AOCI, net of tax	7.5
Ending balance	(136.7)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(144.5)
Other comprehensive income before reclassifications, net of tax	(0.2)
Amounts reclassified from AOCI, net of tax	7.9
Ending balance	(136.8)
Foreign Exchange Contract | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(0.2)
Other comprehensive income before reclassifications, net of tax	0.0
Amounts reclassified from AOCI, net of tax	0.0
Ending balance	(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]
Beginning balance	0.3
Other comprehensive income before reclassifications, net of tax	(0.2)
Amounts reclassified from AOCI, net of tax	0.0
Ending balance	$ 0.1