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Investments - Additional Information (Detail) | 3 Months Ended |
--- | --- | ---
Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($)
Reported Value Measurement [Member] |  |
Investment [Line Items] |  |
Other Investments | $ 106,000,000 | $ 89,000,000
Reported Value Measurement [Member] | Interest Rate Contract [Member] |  |
Investment [Line Items] |  |
Other Investments | 0 | 3,000,000
Reported Value Measurement [Member] | Equity [Member] |  |
Investment [Line Items] |  |
Other Investments | 82,000,000 | 60,000,000
Reported Value Measurement [Member] | Emerging Market Debt [Member] |  |
Investment [Line Items] |  |
Other Investments | 24,000,000 | 25,000,000
Other Investments | 690,000,000 | 595,000,000
Securities fair value | 17,000,000 | 8,000,000
Market Value of Secured Note Collateral | 0 | 178,000,000
Fair Value of Securities Deposited in Connection wtih Letter of Credit | 1,000,000 |
Debt Securities, Available-for-sale, Allowance for Credit Loss | 0 | 0
Equity Securities Excluded From Fair Value | 8,000,000 | 0
Real Estate [Member] |  |
Investment [Line Items] |  |
Other Investments | 33,000,000 | 16,000,000
Hedge Funds, Multi-strategy [Member] |  |
Investment [Line Items] |  |
Other Investments | 216,000,000 | 196,000,000
Interest Rate Contract [Member] |  |
Investment [Line Items] |  |
Other Investments | 78,000,000 | 78,000,000
Illiquid Investments [Member] |  |
Investment [Line Items] |  |
Other Investments | 88,000,000 | 65,000,000
Insurance Linked [Member] |  |
Investment [Line Items] |  |
Other Investments | 2,000,000 | 3,000,000
Equity [Member] |  |
Investment [Line Items] |  |
Other Investments | 98,000,000 | 73,000,000
Credit Index Product [Member] |  |
Investment [Line Items] |  |
Other Investments | 107,000,000 | 73,000,000
Equity investments in pooled funds [Member] |  |
Investment [Line Items] |  |
Other Investments | 683,000,000 | 543,000,000
Private Equity Funds, US |  |
Investment [Line Items] |  |
Other Investments | 37,000,000 | 13,000,000
Emerging Markets Debt |  |
Investment [Line Items] |  |
Other Investments | 24,000,000 | 25,000,000
Investment Portfolio [Member] |  |
Investment [Line Items] |  |
Fair Value of Cash and Securities Pledged to Derivative Counterparties | $ 120,000,000 | $ 140,000,000
Ambac Assurance Corporation [Member] | Corolla Notes | 5.1% Junior Surplus Notes, General Account Due 2020 [Member] |  |
Investment [Line Items] |  |
Note Holders | 1 |
Commitments [Member] | Partnership Interest [Member] |  |
Investment [Line Items] |  |
Other Investments | $ 74,000,000 |

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