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Mortgage Servicing Rights - 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
Transfers and Servicing of Financial Assets [Abstract]          
Residential mortgage loans serviced for others including subserviced mortgages with no corresponding MSRs asset $ 214,600   $ 214,600   $ 226,000
Gain (Loss) on fair value changes of MSRs due to changes in valuation assumptions and derivatives used to economically hedge MSRs 9 $ (2) 58 $ (5)  
Loan servicing and ancillary fees $ 177 $ 188 $ 537 $ 547