RECLASSIFICATIONS OUT OF ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income	$ 1,477	$ 1,264	$ 4,216	$ 3,629
Interest expense	(329)	(202)	(856)	(536)
Securities gains, net	3	2	13	9
Net debt securities impairment losses recognized in earnings	(1)	(1)	(3)	(6)
Other operating expense	(131)	(133)	(378)	(405)
Income before income tax expense	576	513	1,626	1,409
Income tax expense	133	165	370	423
NET INCOME	443	348	1,256	986
Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	(4)	0	(7)	3
NET INCOME	(13)	0	(21)	7
Reclassification adjustment for net derivative (losses) gains included in net income: | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income	(17)	3	(36)	23
Interest expense	3	1	11	(2)
Income before income tax expense	(14)	4	(25)	21
Income tax expense	(3)	2	(6)	8
NET INCOME	(11)	2	(19)	13
Reclassification of net debt securities gains (losses) to net income: | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Securities gains, net	3	2	13	9
Net debt securities impairment losses recognized in earnings	(1)	(1)	(3)	(6)
Income before income tax expense	2	1	10	3
Income tax expense	0	0	2	1
NET INCOME	2	1	8	2
Reclassification of changes related to the employee benefit plan: | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Other operating expense	(5)	(5)	(13)	(14)
Income before income tax expense	(5)	(5)	(13)	(14)
Income tax expense	(1)	(2)	(3)	(6)
NET INCOME	$ (4)	$ (3)	$ (10)	$ (8)