MORTGAGE BANKING - Economic Assumptions Used to Estimate Value of MSRs (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Weighted average life (in years)	5 years 2 months 12 days	5 years 2 months 12 days
Weighted average constant prepayment rate (percent)	12.30%	12.40%
Weighted average discount rate (percent)	9.70%	9.80%
Residential mortgages
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Fair value	$ 177	$ 179
Weighted average life (in years)	5 years 2 months 12 days	5 years 2 months 12 days
Weighted average constant prepayment rate (percent)	12.30%	12.40%
Weighted average discount rate (percent)	9.80%	9.80%