CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Change in Each Component of AOCI (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in accumulated other comprehensive income (loss)
Beginning-of-period balance, net of tax		$ (32,381)
Impact of Tax Reform related to AOCI reclassification		(3,304)
Other comprehensive income (losses) before reclassifications		726	$ (2,903)	$ (6,220)
Increase (decrease) due to amounts reclassified from AOCI		(213)	(119)	92
Change, net of taxes		(2,791)	(3,022)	(6,128)
End-of-period balance, net of tax		(34,668)	(32,381)
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss)
Beginning-of-period balance, net of tax		(799)	(907)	57
Adjustment to opening balance, net of taxes			504	0
Adjusted balance, beginning of period			(295)	(907)
Impact of Tax Reform related to AOCI reclassification		(223)
Other comprehensive income (losses) before reclassifications		(186)	530	(695)
Increase (decrease) due to amounts reclassified from AOCI		(454)	(422)	(269)
Change, net of taxes		(863)	108	(964)
End-of-period balance, net of tax		(1,158)	(799)	(907)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss)
Beginning-of-period balance, net of tax		(352)	0	0
Adjustment to opening balance, net of taxes			0	(15)
Adjusted balance, beginning of period			(352)	(15)
Impact of Tax Reform related to AOCI reclassification		(139)
Other comprehensive income (losses) before reclassifications		(426)	(335)	0
Increase (decrease) due to amounts reclassified from AOCI		(4)	(2)	0
Change, net of taxes		(569)	(337)	0
End-of-period balance, net of tax		(921)	(352)	0
Cash flow hedges
Change in accumulated other comprehensive income (loss)
Beginning-of-period balance, net of tax		(560)	(617)	(909)
Adjustment to opening balance, net of taxes			0	0
Adjusted balance, beginning of period			(560)	(617)
Impact of Tax Reform related to AOCI reclassification		(113)
Other comprehensive income (losses) before reclassifications		(111)	(88)	83
Increase (decrease) due to amounts reclassified from AOCI		86	145	209
Change, net of taxes		(138)	57	292
End-of-period balance, net of tax		(698)	(560)	(617)
Benefit plans
Change in accumulated other comprehensive income (loss)
Beginning-of-period balance, net of tax		(5,164)	(5,116)	(5,159)
Adjustment to opening balance, net of taxes			0	0
Adjusted balance, beginning of period			(5,164)	(5,116)
Impact of Tax Reform related to AOCI reclassification		(1,020)	0	0
Other comprehensive income (losses) before reclassifications		(158)	(208)	(143)
Increase (decrease) due to amounts reclassified from AOCI		159	160	186
Change, net of taxes		(1,019)	(48)	43
End-of-period balance, net of tax		(6,183)	(5,164)	(5,116)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss)
Beginning-of-period balance, net of tax		(25,506)	(22,704)	(17,205)
Adjustment to opening balance, net of taxes			0	0
Adjusted balance, beginning of period			(25,506)	(22,704)
Impact of Tax Reform related to AOCI reclassification		(1,809)
Other comprehensive income (losses) before reclassifications		1,607	(2,802)	(5,465)
Increase (decrease) due to amounts reclassified from AOCI		0	0	(34)
Change, net of taxes		(202)	(2,802)	(5,499)
End-of-period balance, net of tax		(25,708)	(25,506)	(22,704)
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss)
Beginning-of-period balance, net of tax		(32,381)	(29,344)	(23,216)
Adjustment to opening balance, net of taxes	[1]		504	(15)
Adjusted balance, beginning of period			(31,877)	(29,359)	$ (23,216)
Increase (decrease) due to amounts reclassified from AOCI		(213)	(119)	92
Change, net of taxes	[2]	(2,791)	(3,022)	(6,128)
End-of-period balance, net of tax		$ (34,668)	$ (32,381)	$ (29,344)

[1] See Note 1 to the Consolidated Financial Statements.

[2] Includes the impact of ASU 2018-02, which transferred those amounts from AOCI to Retained earnings. See Notes 1 and 19 to the Consolidated Financial Statements.