Investments - Summary of Amortized Cost and Estimated Fair Value of Available-for-Sale Investments, Excluding VIE Investments Held by Successor Ambac, by Contractual Maturity (Detail) - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Debt Securities, Available-for-sale [Line Items]
Amortized Cost, Due in one year or less	$ 544
Amortized Cost, Due after one year through five years	581
Amortized Cost, Due after five years through ten years	293
Amortized Cost, Due after ten years	80
Amortized Cost, Total	1,500
Fixed income securities, amortized cost	2,197	$ 2,041
Estimated Fair Value, Due in one year or less	544
Estimated Fair Value, Due after one year through five years	560
Estimated Fair Value, Due after five years through ten years	271
Estimated Fair Value, Due after ten years	76
Estimated Fair Value due, Total	1,451
Debt Securities, Available-for-sale	1,710	1,395
Residential Mortgage-Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	239	230
Debt Securities, Available-for-sale	250	238
Collateralized Debt Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	139	141
Debt Securities, Available-for-sale	139	137
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Fixed income securities, amortized cost	301	227
Debt Securities, Available-for-sale	$ 303	$ 224