MORTGAGE BANKING - Economic Assumptions Used to Estimate Value of MSRs (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Weighted average life (in years)	4 years 10 months 24 days	5 years 4 months 24 days
Weighted average constant prepayment rate	13.30%	11.60%
Weighted average discount rate	9.70%	9.70%
Residential mortgages
Servicing Assets and Servicing Liabilities at Fair Value, Assumptions Used to Estimate Fair Value [Abstract]
Fair value	$ 168	$ 178
Weighted average life (in years)	4 years 10 months 24 days	5 years 4 months 24 days
Weighted average constant prepayment rate	13.30%	11.60%
Weighted average discount rate	9.70%	9.70%