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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, 2021
Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost, Due in one year or less $ 565  
Amortized Cost, Due after one year through five years 457  
Amortized Cost, Due after five years through ten years 406  
Amortized Cost, Due after ten years 168  
Amortized Cost, Total 1,596  
Fixed income securities, amortized cost 2,140  
Estimated Fair Value, Due in one year or less 565  
Estimated Fair Value, Due after one year through five years 457  
Estimated Fair Value, Due after five years through ten years 411  
Estimated Fair Value, Due after ten years 188  
Estimated Fair Value due, Total 1,620  
Debt Securities, Available-for-sale 2,265  
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 1,605 $ 2,175
Debt Securities, Available-for-sale 1,730 2,317
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 182  
Debt Securities, Available-for-sale 252 302
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 128  
Debt Securities, Available-for-sale 128 74
Fixed Income Securities [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 234  
Debt Securities, Available-for-sale $ 265 $ 303