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Borrowings - Additional Information (Detail) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Jul. 17, 2020
Jan. 07, 2020
Sep. 30, 2020
Dec. 31, 2019
Debt Instrument [Line Items]        
Junior subordinated debentures     $ 527  
Debt Maturity, Start Year     Jan. 01, 2027  
Debt Maturity, End Year     Dec. 31, 2033  
Agreements to repurchase securities       $ 102
Collateral posted       $ 108
Fixed Rate Senior Notes Due On February 6, 2020 [Member]        
Debt Instrument [Line Items]        
Redeemed senior notes   $ 750    
Maturity Date   Aug. 17, 2020    
Fixed Rate Senior Notes Due On July 17, 2020 [Member]        
Debt Instrument [Line Items]        
Redeemed senior notes $ 750      
Maturity Date Aug. 17, 2020