Investments - Available-for-Sale Debt Securities (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 3,221.7	$ 2,661.4
Gross Unrealized Gains	0.1	2.6
Gross Unrealized Losses	(88.0)	(9.3)
Fair Value	3,133.8	2,654.7
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,665.8	2,178.9
Gross Unrealized Gains	0.1	2.0
Gross Unrealized Losses	(68.7)	(7.3)
Fair Value	2,597.2	2,173.6
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	555.9	482.5
Gross Unrealized Gains	0.0	0.6
Gross Unrealized Losses	(19.3)	(2.0)
Fair Value	$ 536.6	$ 481.1

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

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 (c)
-URI https://asc.fasb.org/extlink&oid=126970911&loc=d3e27161-111563

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Amortized cost excluding accrued interest, after 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/2009/role/commonPracticeRef
-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).

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