Investments - Summary of Amounts Included in Net Realized (Losses) Gains and Other-Than-Temporary Impairments (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Investment [Line Items]
Gross realized gains on securities	$ 1	$ 5	$ 9	$ 32
Gross realized losses on securities	0	(1)	(2)	(12)
Foreign exchange (losses) gains			(6)	2
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not to Sell before Recovery, Credit Loss, Previously Recorded, Expense (Reversal)	0	0	0	0
Debt Securities, Available-for-sale, Allowance for Credit Loss, Sell before Recovery	0	0	0	0
Net realized (losses) gains	3	2	4	20
Fair Value of Securities Deposited in Connection wtih Letter of Credit	1		1		$ 1
Gain (Loss) on Investments [Member]
Investment [Line Items]
Foreign exchange (losses) gains	$ 3	$ (2)	$ (3)	$ 0

Fair Value of Securities Deposited in Connection wtih Letter of Credit

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This item represents the difference between the gross realized gains and losses realized on the sale of debt or equity securities categorized neither as held-to-maturity nor trading securities.

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Amount of expense (reversal of expense) for allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), not intended to be sold and not more likely than not will be required to sell before recovery of amortized cost basis, with previously recorded allowance.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
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Amount of decrease in allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale) from intent to sell or more likely than not will be required to sell before recovery of amortized cost basis.

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Amount before tax of foreign currency transaction realized and unrealized gain (loss) recognized in the income statement.

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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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