Investments - Additional Information (Detail)	3 Months Ended
	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Reported Value Measurement [Member]
Investment [Line Items]
Other Investments	$ 61,000,000	$ 106,000,000
Reported Value Measurement [Member] | Equity [Member]
Investment [Line Items]
Other Investments	53,000,000	82,000,000
Reported Value Measurement [Member] | Emerging Market Debt [Member]
Investment [Line Items]
Other Investments	0	24,000,000
Other Investments	568,000,000	690,000,000
Securities fair value	23,000,000	17,000,000
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	12,000,000	8,000,000
Real Estate [Member]
Investment [Line Items]
Other Investments	22,000,000	33,000,000
Hedge Funds, Multi-strategy [Member]
Investment [Line Items]
Other Investments	186,000,000	216,000,000
Interest Rate Contract [Member]
Investment [Line Items]
Other Investments	80,000,000	78,000,000
Illiquid Investments [Member]
Investment [Line Items]
Other Investments	84,000,000	88,000,000
Insurance Linked [Member]
Investment [Line Items]
Other Investments	1,000,000	2,000,000
Equity [Member]
Investment [Line Items]
Other Investments	64,000,000	98,000,000
Credit Index Product [Member]
Investment [Line Items]
Other Investments	63,000,000	107,000,000
Equity investments in pooled funds [Member]
Investment [Line Items]
Other Investments	556,000,000	683,000,000
Private Equity Funds, US
Investment [Line Items]
Other Investments	47,000,000	37,000,000
Emerging Markets Debt
Investment [Line Items]
Other Investments	0	24,000,000
Convertibles and Bonds with Warrants Attached
Investment [Line Items]
Other Investments	8,000,000	0
Investment Portfolio [Member]
Investment [Line Items]
Fair Value of Cash and Securities Pledged to Derivative Counterparties	$ 64,000,000	$ 120,000,000
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	$ 53,000,000