Investments - Additional Information (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0
Other investments	363,000,000	$ 478,000,000
Securities fair value	7,000,000	6,000,000
Ambac Assurance [Member]
Schedule of Investments [Line Items]
Unsettled Amounts Secured Notes from LSNI	31,000,000	55,000,000
Interest Rate Contract [Member]
Schedule of Investments [Line Items]
Other investments	57,000,000	$ 176,000,000
Commitments [Member] | Partnership Interest [Member]
Schedule of Investments [Line Items]
Other investments	$ 76,000,000