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MORTGAGE BANKING AND OTHER - Economic Assumptions Used to Estimate Value of MSRs (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]    
Fair value $ 658 $ 642
Weighted-average life (in years) 4 years 2 months 12 days 5 years 6 months
Weighted-average constant prepayment rate 17.30% 13.90%
Weighted average option adjusted spread 5.95% 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 $ 122 $ 116
Decline in fair value due to 50 bps decrease in option adjusted spread $ 12 $ 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 $ 202 $ 222
Decline in fair value due to 100 bps decrease in option adjusted spread $ 24 $ 25