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Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	$ 478		$ 391
Credit valuation adjustment included in fair value of credit derivative liabilities	0		0
Fair value of derivative liabilities	89
Derivative liabilities	$ 90		$ 77
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.40%		2.70%
Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using dealer quotes	4.00%		8.00%
Asset-backed Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Coupon rate	5.97%		5.97%
European ABS Transactions [Member] \| Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value of consolidated VIE debt obligations valued using unobservable inputs by asset class	$ 0		$ 217
Minimum [Member] \| Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using internal valuation models	2.00%		2.00%
Maximum [Member] \| Fixed Income Securities [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Percentage of investment portfolio valued using external pricing services	94.00%		90.00%
Variable Interest Entity, Primary Beneficiary [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Fair value of derivative liabilities	$ 0
Derivative liabilities	$ 1,657		$ 1,712
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs	2.70%		3.10%
Variable Interest Entity, Primary Beneficiary [Member] \| European ABS Transactions [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Coupon rate			2.20%
Real Estate [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	$ 16		$ 16
Hedge Funds, Multi-strategy [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments		$ 65	0
Interest Rate Contract [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	176		114
Illiquid Investments [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Other Investments	$ 51		84
Other Credit Derivatives [Member] \| Level 3 [Member]
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items]
Derivative liabilities			$ 1