Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 698.6	$ 6.4
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(1.1)	0.0
Fair Value	697.6	6.6
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	588.0	0.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	(0.9)	0.0
Fair Value	587.1	0.0
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	110.0	6.4
Gross Unrealized Gains	0.1	0.2
Gross Unrealized Losses	(0.2)	0.0
Fair Value	109.9	6.6
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	0.6	0.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 0.6	$ 0.0

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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