EXIT COSTS AND RESTRUCTURING RESERVES - Reserve Rollforward (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Restructuring Reserve [Roll Forward]
Beginning balance	$ 33	$ 44
Additions		31
Reversals		(5)
Utilization	(9)	(37)
Ending balance	24	33
Salaries & Employee Benefits | Salaries & Employee Benefits
Restructuring Reserve [Roll Forward]
Beginning balance	12	23
Additions		5
Reversals		(4)
Utilization	(2)	(12)
Ending balance	10	12
Occupancy & Equipment | Occupancy & Equipment
Restructuring Reserve [Roll Forward]
Beginning balance	16	18
Additions		18
Reversals		(1)
Utilization	(2)	(19)
Ending balance	14	16
Other | Other
Restructuring Reserve [Roll Forward]
Beginning balance	5	3
Additions		8
Reversals		0
Utilization	(5)	(6)
Ending balance	$ 0	$ 5