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Fair Values of Assets and Liabilities - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Fair Value Disclosures [Abstract]          
Carrying value of unfunded commitments, deferred non-yield related loan fees and standby letters of credit $ 681   $ 681   $ 528
Other guarantees carrying value 352   352   $ 200
Changes to fair value of these MLHFS $ 97 $ 10 $ 271 $ 53  
Minimum [Member]          
Fair Value Disclosures [Abstract]          
Assumed credit valuation adjustment as percentage of derivative contract fair value     0.00%    
Maximum [Member]          
Fair Value Disclosures [Abstract]          
Assumed credit valuation adjustment as percentage of derivative contract fair value     101.00%    
Weighted Average [Member]          
Fair Value Disclosures [Abstract]          
Assumed credit valuation adjustment as percentage of derivative contract fair value     2.00%