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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, 2019
Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]    
Amortized Cost, Due in one year or less $ 749  
Amortized Cost, Due after one year through five years 1,157  
Amortized Cost, Due after five years through ten years 477  
Amortized Cost, Due after ten years 145  
Amortized Cost, Total 2,528  
Fixed income securities, amortized cost 3,187 $ 3,451
Estimated Fair Value, Due in one year or less 749  
Estimated Fair Value, Due after one year through five years 1,173  
Estimated Fair Value, Due after five years through ten years 501  
Estimated Fair Value, Due after ten years 160  
Estimated Fair Value due, Total 2,583  
Debt Securities, Available-for-sale 3,314  
Fixed Income Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 2,450 3,021
Debt Securities, Available-for-sale 2,577 3,116
Fixed Income Securities [Member] | Residential Mortgage-Backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 200 222
Debt Securities, Available-for-sale 248 259
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 147 133
Debt Securities, Available-for-sale 146 131
Fixed Income Securities [Member] | Asset-backed Securities [Member]    
Debt Securities, Available-for-sale [Line Items]    
Fixed income securities, amortized cost 263 370
Debt Securities, Available-for-sale $ 287 $ 442