Investments - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Investment [Line Items]
Other Investments	$ 478		$ 391
Securities fair value	6		6
Market Value of Secured Note Collateral	197		210
Real Estate [Member]
Investment [Line Items]
Other Investments	16		16
Hedge Funds, Multi-strategy [Member]
Investment [Line Items]
Other Investments		$ 65	0
Interest Rate Contract [Member]
Investment [Line Items]
Other Investments	176		114
Illiquid Investments [Member]
Investment [Line Items]
Other Investments	51		84
Insurance Linked [Member]
Investment [Line Items]
Other Investments	3		29
Equity [Member]
Investment [Line Items]
Other Investments	55		44
Credit Index Product [Member]
Investment [Line Items]
Other Investments	66		63
Equity investments in pooled funds [Member]
Investment [Line Items]
Other Investments	432			$ 351
Reported Value Measurement [Member] | Interest Rate Contract [Member]
Investment [Line Items]
Other Investments	81		27
Priced Through Pricing Vendors [Member] | Equity [Member]
Investment [Line Items]
Other Investments	55		44
Investment Portfolio [Member]
Investment [Line Items]
Fair Value of Cash and Securities Pledged to Derivative Counterparties	85		103
Ambac Assurance [Member]
Investment [Line Items]
Unsettled Amounts Secured Notes from LSNI	55		$ 19
Commitments [Member] | Partnership Interest [Member]
Investment [Line Items]
Other Investments	41
Commitments [Member] | Capital of Dislocations [Member]
Investment [Line Items]
Other Investments	$ 48