Quarterly report pursuant to Section 13 or 15(d)

MORTGAGE LOAN SALES AND SERVICING RIGHTS - Residential Mortgage Portfolio, MSRs Fair Value Method Key Assumptions (Details)

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MORTGAGE LOAN SALES AND SERVICING RIGHTS - Residential Mortgage Portfolio, MSRs Fair Value Method Key Assumptions (Details) - USD ($)
$ in Millions
3 Months Ended 9 Months Ended 12 Months Ended
Mar. 31, 2020
Sep. 30, 2020
Dec. 31, 2019
Dec. 31, 2018
Servicing Assets at Fair Value [Line Items]        
Spread over forward interest rate swap rates, actual   7.98% 8.24%  
Fair value method | Residential Mortgage        
Servicing Assets at Fair Value [Line Items]        
Constant prepayment rate (annualized), actual 8.21% 15.62%    
Constant prepayment rate (annualized), 10% adverse change   $ (9) $ 0  
Constant prepayment rate (annualized), 20% adverse change   (18)   $ 0
Spread over forward interest rate swap rates, 10% adverse change   (5) $ 0  
Spread over forward interest rate swap rates, 20% adverse change   $ (11)   $ 0