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MORTGAGE BANKING - Economic Assumptions Used to Estimate Value of MSRs (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Servicing Assets And Servicing Liabilities At Amortized Cost, Assumptions Used To Estimate Fair Value [Abstract]    
MRS portfolio fair value $ 261 $ 218
Weighted average life (in years) 6 years 10 months 24 days 5 years 10 months 24 days
Weighted average constant prepayment rate 7.70% 10.00%
Weighted average discount rate 9.30% 9.90%
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]    
MSR portfolio fair value $ 612  
Weighted average life (in years) 8 years 8 months 12 days  
Weighted average constant prepayment rate 7.30%  
Weighted average option adjusted spread 6.25%  
Minimum    
Servicing Assets at Fair Value [Line Items]    
Sensitivity analysis, basis spread 0.50% 0.50%
Servicing Assets And Servicing Liabilities At Amortized Cost, Assumptions Used To Estimate Fair Value [Abstract]    
Decline in fair value due to 50 bps decrease in prepayment rate $ 18 $ 22
Decline in fair value due to 50 bps decrease in discount rate 5 $ 4
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 54  
Decline in fair value due to 50 bps decrease in discount rate $ 14  
Maximum    
Servicing Assets at Fair Value [Line Items]    
Sensitivity analysis, basis spread 1.00% 1.00%
Servicing Assets And Servicing Liabilities At Amortized Cost, Assumptions Used To Estimate Fair Value [Abstract]    
Decline in fair value due to 100 bps decrease in prepayment rate $ 43 $ 46
Decline in fair value due to 100 bps decrease in discount rate 10 $ 8
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 123  
Decline in fair value due to 100 bps decrease in discount rate $ 27