FAIR VALUE MEASUREMENTS - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Mortgage banking (expense) income		$ 101,000,000	$ 71,000,000	$ 153,000,000
Transfers from Level 3 to Level 2	$ 5,000,000
Weighted average life (in years)		5 years 4 months 24 days	5 years 2 months 12 days
Weighted average constant prepayment rate		11.60%	12.40%
Weighted average discount rate		9.70%	9.80%
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Weighted average life (in years)		2 years 9 months 18 days	2 years 9 months 18 days
Weighted average constant prepayment rate		10.70%	10.40%
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)		6 years 2 months 12 days	6 years 7 months 6 days
Weighted average constant prepayment rate		22.20%	22.60%
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		$ (2,000,000)	$ 5,000,000	(33,000,000)
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		$ 3,000,000	$ 1,000,000	$ 0