RECLASSIFICATIONS OUT OF ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income	$ 1,570	$ 1,328
Interest expense	(410)	(237)
Securities gains, net	8	8
Net debt securities impairment losses recognized in earnings	(1)	(1)
Other operating expense	(110)	(120)
Income before income tax expense	566	501
Income tax expense	127	113
NET INCOME	439	388
Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	(5)	1
NET INCOME	(13)	0
Reclassification adjustment for net derivative losses included in net income: | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income	(20)	(6)
Interest expense	0	4
Income before income tax expense	(20)	(2)
Income tax expense	(5)	0
NET INCOME	(15)	(2)
Reclassification of net debt securities gains to net income: | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Securities gains, net	8	8
Net debt securities impairment losses recognized in earnings	(1)	(1)
Income before income tax expense	7	7
Income tax expense	2	2
NET INCOME	5	5
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)	(4)
Income before income tax expense	(5)	(4)
Income tax expense	(2)	(1)
NET INCOME	$ (3)	$ (3)