Investments - Amortized Cost and Fair Values (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 665.7	$ 673.5
Gross unrealized gains	0.7	0.1
Gross unrealized losses	(17.9)	(23.3)
Fair Value	648.5	650.3
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	560.0	549.7
Gross unrealized gains	0.6	0.1
Gross unrealized losses	(15.2)	(19.6)
Fair Value	545.4	530.2
U.S. Government obligations
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	103.9	121.0
Gross unrealized gains	0.1	0.0
Gross unrealized losses	(2.7)	(3.7)
Fair Value	101.3	117.3
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1.8	2.8
Gross unrealized gains	0.0	0.0
Gross unrealized losses	0.0	0.0
Fair Value	$ 1.8	$ 2.8

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).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

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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).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
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Amortized cost excluding accrued interest, before allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI https://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

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Amount excluding accrued interest, 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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-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
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