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Investments - Additional Information (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019
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Reported Value Measurement [Member] |  |
Investment [Line Items] |  |
Other Investments | $ 89 | $ 136
Reported Value Measurement [Member] | Interest Rate Contract [Member] |  |
Investment [Line Items] |  |
Other Investments | 3 | 81
Reported Value Measurement [Member] | Equity [Member] |  |
Investment [Line Items] |  |
Other Investments | 60 | 55
Reported Value Measurement [Member] | Emerging Market Debt [Member] |  |
Investment [Line Items] |  |
Other Investments | 25 | 0
Other Investments | 595 | 478
Securities fair value | 8 | 6
Market Value of Secured Note Collateral | 178 | 197
Fair Value of Securities Deposited in Connection wtih Letter of Credit | 1 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 |
Real Estate [Member] |  |
Investment [Line Items] |  |
Other Investments | 16 | 16
Hedge Funds, Multi-strategy [Member] |  |
Investment [Line Items] |  |
Other Investments | 196 | 65
Interest Rate Contract [Member] |  |
Investment [Line Items] |  |
Other Investments | 78 | 176
Illiquid Investments [Member] |  |
Investment [Line Items] |  |
Other Investments | 65 | 51
Insurance Linked [Member] |  |
Investment [Line Items] |  |
Other Investments | 3 | 3
Equity [Member] |  |
Investment [Line Items] |  |
Other Investments | 73 | 55
Credit Index Product [Member] |  |
Investment [Line Items] |  |
Other Investments | 73 | 66
Equity investments in pooled funds [Member] |  |
Investment [Line Items] |  |
Other Investments | 543 | 432
Investment Portfolio [Member] |  |
Investment [Line Items] |  |
Fair Value of Cash and Securities Pledged to Derivative Counterparties | 140 | $ 85
Commitments [Member] | Partnership Interest [Member] |  |
Investment [Line Items] |  |
Other Investments | $ 81 |

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