Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments (Detail) - USD ($)	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	$ 2,197,000,000	$ 2,041,000,000
Gross Unrealized Gains	40,000,000	31,000,000
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	71,000,000	106,000,000
Available-for-sale Securities	1,710,000,000	1,395,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	3,000,000	0
Debt Securities, Available-for-Sale, Current	2,162,000,000	1,966,000,000
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	1,744,000,000	1,469,000,000
Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	72,000,000	44,000,000
Gross Unrealized Gains	1,000,000	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,000,000	2,000,000
Available-for-sale Securities	72,000,000	43,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Corporate Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	785,000,000	659,000,000
Gross Unrealized Gains	4,000,000	1,000,000
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	44,000,000	63,000,000
Available-for-sale Securities	745,000,000	598,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt Security, Government, Non-US [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	105,000,000	85,000,000
Gross Unrealized Gains	1,000,000	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	6,000,000	9,000,000
Available-for-sale Securities	100,000,000	76,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
U.S. Government Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	85,000,000	68,000,000
Gross Unrealized Gains	1,000,000	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	4,000,000	4,000,000
Available-for-sale Securities	82,000,000	65,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	239,000,000	230,000,000
Gross Unrealized Gains	28,000,000	28,000,000
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	14,000,000	19,000,000
Available-for-sale Securities	250,000,000	238,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	3,000,000	0
Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	139,000,000	141,000,000
Gross Unrealized Gains	1,000,000	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,000,000	4,000,000
Available-for-sale Securities	139,000,000	137,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	301,000,000	227,000,000
Gross Unrealized Gains	3,000,000	2,000,000
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	1,000,000	5,000,000
Available-for-sale Securities	303,000,000	224,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	426,000,000	507,000,000
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Available-for-sale Securities	426,000,000	507,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Commercial Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	19,000,000	15,000,000
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Available-for-sale Securities	19,000,000	0
Debt Securities, Available-for-sale, Allowance for Credit Loss	0
Accumulated Other Comprehensive Income (Loss), Other than Temporary Impairment, Not Credit Loss, Net of Tax, Available-for-sale, Debt Securities		15,000,000
Collateralized Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	19,000,000
Fixed Income Investments And Other Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	2,170,000,000	1,977,000,000
Gross Unrealized Gains	40,000,000	31,000,000
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	71,000,000	106,000,000
Available-for-sale Securities	2,135,000,000	1,902,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	3,000,000	0
Collateral Pledged [Member]
Debt Securities, Available-for-sale [Line Items]
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Available-for-sale Securities	27,000,000	64,000,000
Collateral Pledged [Member] | Short-term Investments [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	27,000,000	64,000,000
Gross Unrealized Gains	0	0
Available For Sale Securities Gross Unrealized Loss Accumulated In Investments	0	0
Available-for-sale Securities	27,000,000	64,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Asset Pledged as Collateral [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	27,000,000	64,000,000
Debt Securities, Available-for-sale, Allowance for Credit Loss	$ 0	$ 0