RECLASSIFICATIONS OUT OF ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income	$ 1,580	$ 1,411	$ 3,150	$ 2,739
Interest expense	(414)	(290)	(824)	(527)
Securities gains, net	4	2	12	10
Net debt securities impairment losses recognized in earnings	0	(1)	(1)	(2)
Other operating expense	(118)	(127)	(228)	(247)
Income before income tax expense	580	549	1,146	1,050
Income tax expense	127	124	254	237
NET INCOME	453	425	892	813
Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense	(4)	(4)	(9)	(3)
NET INCOME	(15)	(8)	(28)	(8)
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)	(13)	(40)	(19)
Interest expense	1	4	1	8
Income before income tax expense	(19)	(9)	(39)	(11)
Income tax expense	(4)	(3)	(9)	(3)
NET INCOME	(15)	(6)	(30)	(8)
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	4	2	12	10
Net debt securities impairment losses recognized in earnings	0	(1)	(1)	(2)
Income before income tax expense	4	1	11	8
Income tax expense	1	0	3	2
NET INCOME	3	1	8	6
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	(4)	(4)	(9)	(8)
Income before income tax expense	(4)	(4)	(9)	(8)
Income tax expense	(1)	(1)	(3)	(2)
NET INCOME	$ (3)	$ (3)	$ (6)	$ (6)