Investments -Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Dec. 31, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 673.5	$ 696.8
Gross unrealized gains	0.1	0.1
Gross unrealized losses	(23.3)	(5.5)
Fair Value	650.3	691.4
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	549.7	593.4
Gross unrealized gains	0.1	0.0
Gross unrealized losses	(19.6)	(4.7)
Fair Value	530.2	588.7
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	121.0	102.2
Gross unrealized gains	0.0	0.1
Gross unrealized losses	(3.7)	(0.8)
Fair Value	117.3	101.5
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	2.8	0.0
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair Value	2.8	0.0
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	0.0	1.2
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair Value	$ 0.0	$ 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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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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