Stockholders' Equity - Changes in Accumulated Other Comprehensive Income by Component (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 3,246.3
Net current-period other comprehensive income (loss)	69.3	$ 15.0	$ (20.7)
Ending balance	3,607.2	3,246.3
Foreign Currency Translation Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(0.4)	(0.4)
Other comprehensive income before reclassifications	0.0	0.0
Amounts reclassified from AOCI	(0.4)	0.0
Net current-period other comprehensive income (loss)	0.4	0.0
Ending balance	(0.0)	(0.4)	(0.4)
Unfunded Employee Benefit Obligations
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(144.2)	(158.9)
Other comprehensive income before reclassifications	59.4	4.0
Amounts reclassified from AOCI	10.0	10.7
Net current-period other comprehensive income (loss)	69.4	14.7
Ending balance	(74.8)	(144.2)	(158.9)
Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(144.5)	(159.5)
Other comprehensive income before reclassifications	58.9	4.3
Amounts reclassified from AOCI	10.4	(10.7)
Net current-period other comprehensive income (loss)	69.3	15.0
Ending balance	(75.2)	(144.5)	(159.5)
Unrealized Loss on Foreign Exchange Contracts \| 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)	(0.2)
Other comprehensive income before reclassifications	0.0	0.0
Amounts reclassified from AOCI	0.0	0.0
Net current-period other comprehensive income (loss)	0.0	0.0
Ending balance	(0.2)	(0.2)	(0.2)
Unrealized Loss (Gain) on 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	0.0
Other comprehensive income before reclassifications	(0.5)	0.3
Amounts reclassified from AOCI	0.0	0.0
Net current-period other comprehensive income (loss)	(0.5)	0.3
Ending balance	$ (0.2)	$ 0.3	$ 0.0

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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