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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, 2020
Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost, Due in one year or less $ 718  
Amortized Cost, Due after one year through five years 873  
Amortized Cost, Due after five years through ten years 439  
Amortized Cost, Due after ten years 184  
Amortized Cost, Total 2,214  
Fixed income securities, amortized cost 2,807 $ 3,187
Estimated Fair Value, Due in one year or less 719  
Estimated Fair Value, Due after one year through five years 880  
Estimated Fair Value, Due after five years through ten years 459  
Estimated Fair Value, Due after ten years 213  
Estimated Fair Value due, Total 2,271  
Debt Securities, Available-for-sale 2,949 3,314
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 2,175 2,450
Debt Securities, Available-for-sale 2,317 2,577
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 256 200
Debt Securities, Available-for-sale 302 248
Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost   49
Debt Securities, Available-for-sale   50
Fixed Income Securities [Member] | Collateralized Debt Obligations [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 74 147
Debt Securities, Available-for-sale 74 146
Fixed Income Securities [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 263 263
Debt Securities, Available-for-sale $ 303 $ 287