RECLASSIFICATIONS OUT OF ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Reclassifications out of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Interest income		$ 5,758	$ 4,920	$ 4,266
Interest expense		(1,226)	(747)	(508)
Securities gains, net		19	11	16
Net debt securities impairment losses recognized in earnings		(3)	(7)	(12)
Salaries and employee benefits		(495)	(542)	(521)
Income before income tax expense		2,183	1,912	1,534
Income tax expense		462	260	489
NET INCOME		1,721	1,652	1,045
Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense		5	(6)	38
NET INCOME		1	(13)	63
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		(55)	25	90
Interest expense	[1]	12	0	(27)
Other income		0	0	(5)
Income before income tax expense		(43)	25	58
Income tax expense		(10)	9	22
NET INCOME		(33)	16	36
Reclassification of net securities gains (losses) to net income: | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Securities gains, net		19	11	16
Net debt securities impairment losses recognized in earnings		(3)	(7)	(12)
Income before income tax expense		16	4	4
Income tax expense		4	2	2
NET INCOME		12	2	2
Reclassification of changes related to the employee benefit plan: | Amount Reclassified from AOCI
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Salaries and employee benefits		33	(48)	39
Income before income tax expense		33	(48)	39
Income tax expense		11	(17)	14
NET INCOME		$ 22	$ (31)	$ 25

[1] This amount includes both (i) the amortization of effective gains and losses associated with the Company’s terminated cash flow hedges and (ii) the current reporting period’s interest settlements realized on the Company’s active cash flow hedges. Both (i) and (ii) were previously included on the accumulated other comprehensive loss line item on the Consolidated Balance Sheets and were subsequently recorded as adjustments to the interest income or expense of the underlying hedged item.