MORTGAGE BANKING - Sensitivity Analysis (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Minimum
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Adverse change in basis points	0.50%	0.50%
Maximum
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Adverse change in basis points	1.00%	1.00%
Prepayment rate
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Decline in fair value from a 50 basis point decrease in interest rates	$ 16	$ 22
Decline in fair value from a 100 basis point decrease in interest rates	47	46
Weighted average discount rate
Sensitivity Analysis of Fair Value of Interests Continued to be Held by Transferor, Servicing Assets or Liabilities, Impact of Adverse Change in Assumption [Line Items]
Decline in fair value from a 50 basis point increase in weighted average discount rate	5	4
Decline in fair value from a 100 basis point increase in weighted average discount rate	$ 9	$ 8