Investments - Available-for-Sale Debt Securities in Unrealized Loss Position (Details) - USD ($) $ in Millions	Sep. 30, 2022	Dec. 31, 2021
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, Less than 12 months, Fair Value	$ 2,412.5	$ 2,082.2
AFS, Less than 12 months, Gross Unrealized Losses	(60.0)	(9.3)
AFS, 12 months or longer, Fair Value	664.0	0.0
AFS, 12 months or longer, Gross Unrealized Losses	(28.0)	0.0
AFS, Total Fair Value	3,076.5	2,082.2
AFS, Total Gross Unrealized Losses	(88.0)	(9.3)
U.S. government and agency securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, Less than 12 months, Fair Value	1,969.6	1,729.9
AFS, Less than 12 months, Gross Unrealized Losses	(44.9)	(7.3)
AFS, 12 months or longer, Fair Value	570.3	0.0
AFS, 12 months or longer, Gross Unrealized Losses	(23.8)	0.0
AFS, Total Fair Value	2,539.9	1,729.9
AFS, Total Gross Unrealized Losses	(68.7)	(7.3)
Corporate debt securities
Debt Securities, Available-for-sale, Unrealized Loss Position, Accumulated Loss [Abstract]
AFS, Less than 12 months, Fair Value	442.9	352.3
AFS, Less than 12 months, Gross Unrealized Losses	(15.1)	(2.0)
AFS, 12 months or longer, Fair Value	93.7	0.0
AFS, 12 months or longer, Gross Unrealized Losses	(4.2)	0.0
AFS, Total Fair Value	536.6	352.3
AFS, Total Gross Unrealized Losses	$ (19.3)	$ (2.0)

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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), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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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), in continuous unrealized loss position for less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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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), in unrealized loss position without allowance for credit loss.

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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Amount of accumulated unrealized loss on investment in debt security measured at amortized cost (held-to-maturity), in continuous loss position for less than 12 months.

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