Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 714.5	$ 696.8
Gross unrealized gains	0.0	0.1
Gross unrealized losses	(19.8)	(5.5)
Fair Value	694.7	691.4
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	611.2	593.4
Gross unrealized gains	0.0	0.0
Gross unrealized losses	(16.8)	(4.7)
Fair Value	594.4	588.7
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	102.1	102.2
Gross unrealized gains	0.0	0.1
Gross unrealized losses	(3.0)	(0.8)
Fair Value	99.1	101.5
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1.2	1.2
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair Value	$ 1.2	$ 1.2

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).

+ References

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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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