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