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MORTGAGE BANKING - Economic Assumptions Used to Estimate Value of MSRs (Details) - USD ($)
$ in Millions
9 Months Ended
Sep. 30, 2020
Sep. 30, 2019
Dec. 31, 2019
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]      
MSR portfolio fair value $ 606   $ 642
Weighted average life (in years) 3 years 9 months 18 days 5 years 6 months  
Weighted average constant prepayment rate 19.40% 13.90%  
Weighted average option adjusted spread 6.08% 4.40%  
Minimum      
Servicing Assets at Fair Value [Line Items]      
Sensitivity analysis, basis spread 0.50% 0.50%  
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]      
Decline in fair value due to 50 bps decrease in prepayment rate $ 116 $ 116  
Decline in fair value due to 50 bps decrease in option adjusted spread $ 10 $ 12  
Maximum      
Servicing Assets at Fair Value [Line Items]      
Sensitivity analysis, basis spread 1.00% 1.00%  
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]      
Decline in fair value due to 100 bps decrease in prepayment rate $ 169 $ 222  
Decline in fair value due to 100 bps decrease in option adjusted spread $ 21 $ 25