Stockholders' Equity - Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 3,071.0
Other comprehensive income before reclassifications	4.3	$ (40.5)
Amounts reclassified from AOCI	10.7	19.8
Other comprehensive income (loss)	15.0	(20.7)	$ 18.1
Ending balance	3,246.3	3,071.0
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(0.4)	(0.4)
Ending balance	(0.4)	(0.4)	(0.4)
Unfunded Employee Benefit Obligations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(158.9)	(127.9)
Other comprehensive income before reclassifications	4.0	(40.5)
Amounts reclassified from AOCI	10.7	9.5
Other comprehensive income (loss)	14.7	(31.0)
Ending balance	(144.2)	(158.9)	(127.9)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(159.5)	(138.8)
Ending balance	(144.5)	(159.5)	(138.8)
Unrealized Loss on Treasury Locks, Net \| Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance		(10.2)
Amounts reclassified from AOCI		10.2
Other comprehensive income (loss)		10.2
Ending balance			(10.2)
Foreign Exchange Contract \| Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(0.2)	(0.3)
Amounts reclassified from AOCI		0.1
Other comprehensive income (loss)		0.1
Ending balance	(0.2)	$ (0.2)	$ (0.3)
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	0.3
Other comprehensive income (loss)	0.3
Ending balance	$ 0.3

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Amount after tax, before reclassification adjustments, of other comprehensive income (loss), attributable to parent.

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Amount after tax of other comprehensive income (loss) attributable to parent entity.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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