Investments - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
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