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Investments - Summary of Amounts Included in Net Realized (Losses) Gains and Other-Than-Temporary Impairments (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2021 | Mar. 31, 2020 | Dec. 31, 2020
Investment [Line Items] |  |  |
Gross realized gains on securities | $ 7 | $ 6 |
Gross realized losses on securities | (1) | 0 |
Foreign exchange (losses) gains | (5) | 1 |
Debt Securities, Available-for-sale, Allowance for Credit Loss, Not to Sell before Recovery, Credit Loss, Previously Recorded, Expense (Reversal) | 0 | 0 |
Debt Securities, Available-for-sale, Allowance for Credit Loss, Sell before Recovery | 0 | 0 |
Net realized (losses) gains | 2 | 8 |
Fair Value of Securities Deposited in Connection wtih Letter of Credit | 1 |  | $ 1
Gain (Loss) on Investments [Member] |  |  |
Investment [Line Items] |  |  |
Foreign exchange (losses) gains | $ (4) | $ 2 |

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- Definition 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 -Topic 326 -SubTopic 30 -Section 50 -Paragraph 9 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922900-210455 + Details
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