FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage banking (expense) income		$ 18,000,000	$ 33,000,000
Transfers from Level 3 to Level 2	$ 5,000,000
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%
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average life (in years)		2 years 7 months 6 days		2 years 9 months 18 days
Weighted average constant prepayment rate		12.40%		10.70%
Weighted average discount rate		9.10%		9.10%
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average life (in years)		5 years 7 months 6 days		6 years 2 months 12 days
Weighted average constant prepayment rate		22.30%		22.20%
Weighted average discount rate		12.10%		12.10%
Residential loans held for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage banking (expense) income		$ 6,000,000	1,000,000
Commercial real estate loans held for sale
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans in this portfolio that were 90 days or more past due or nonaccruing		0
Other noninterest income		$ 0	$ 1,000,000