CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Change in Each Component of AOCI (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		$ 201,672
Balance, end of period		202,866	$ 228,997
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(1,158)	(799)
Adjustment to opening balance, net of taxes				$ (3)	$ 504
Adjusted balance, beginning of period				(1,161)	(295)
Other comprehensive income before reclassifications		(949)	334
Increase (decrease) due to amounts reclassified from AOCI		(109)	(114)
Total other comprehensive income		(1,058)	220
Balance, end of period		(2,219)	(75)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(921)	(352)
Adjustment to opening balance, net of taxes				0	0
Adjusted balance, beginning of period				(921)	(352)
Other comprehensive income before reclassifications		101	(55)
Increase (decrease) due to amounts reclassified from AOCI		27	(5)
Total other comprehensive income		128	(60)
Balance, end of period		(793)	(412)
Cash flow hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(698)	(560)
Adjustment to opening balance, net of taxes				0	0
Adjusted balance, beginning of period				(698)	(560)
Other comprehensive income before reclassifications		(243)	24
Increase (decrease) due to amounts reclassified from AOCI		21	(26)
Total other comprehensive income		(222)	(2)
Balance, end of period		(920)	(562)
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(6,183)	(5,164)
Adjustment to opening balance, net of taxes				0	0
Adjusted balance, beginning of period				(6,183)	(5,164)
Other comprehensive income before reclassifications		41	(49)
Increase (decrease) due to amounts reclassified from AOCI		47	37
Total other comprehensive income		88	(12)
Balance, end of period		(6,095)	(5,176)
Foreign currency translation adjustment (CTA), net of hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(25,708)	(25,506)
Adjustment to opening balance, net of taxes				0	0
Adjusted balance, beginning of period				(25,708)	(25,506)
Other comprehensive income before reclassifications		1,120	1,465
Increase (decrease) due to amounts reclassified from AOCI		0	(147)
Total other comprehensive income		1,120	1,318
Balance, end of period		(24,588)	(24,188)
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		0	0
Adjustment to opening balance, net of taxes				0	0
Adjusted balance, beginning of period				0	0
Other comprehensive income before reclassifications		(4)	0
Increase (decrease) due to amounts reclassified from AOCI		0	0
Total other comprehensive income		(4)	0
Balance, end of period		(4)	0
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss)
Balance, beginning of period		(34,668)	(32,381)
Adjustment to opening balance, net of taxes	[1]			(3)	504
Adjusted balance, beginning of period				$ (34,671)	$ (31,877)
Other comprehensive income before reclassifications		70	1,719
Increase (decrease) due to amounts reclassified from AOCI		(14)	(255)
Total other comprehensive income		52	1,464
Balance, end of period		$ (34,619)	$ (30,413)

[1] See Note 1 to the Consolidated Financial Statements for additional details.