CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Change in Each Component of AOCI (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017		Jan. 01, 2017	Dec. 31, 2016		Jan. 01, 2016
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		$ 197,074	$ 201,672
Change, net of taxes		852	(2,499)		$ (2,791)
Balance, end of year		193,946	197,074		201,672
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(2,250)	(1,158)		(799)
Adjustment to opening balance, net of taxes							$ (3)			$ 504
Adjusted balance, beginning of period							(1,161)			(295)
Impact of Tax Reform					(223)
Other comprehensive income (losses) before reclassifications		3,065	(866)		(186)
Increase (decrease) due to amounts reclassified from AOCI		(1,080)	(223)		(454)
Change, net of taxes		1,985	(1,089)		(863)
Balance, end of year		(265)	(2,250)		(1,158)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		192	(921)		(352)
Adjustment to opening balance, net of taxes							0			0
Adjusted balance, beginning of period							(921)			(352)
Impact of Tax Reform					(139)
Other comprehensive income (losses) before reclassifications		(1,151)	1,081		(426)
Increase (decrease) due to amounts reclassified from AOCI		15	32		(4)
Change, net of taxes		(1,136)	1,113		(569)
Balance, end of year		(944)	192		(921)
Cash flow hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(728)	(698)		(560)
Adjustment to opening balance, net of taxes							0			0
Adjusted balance, beginning of period							(698)			(560)
Impact of Tax Reform					(113)
Other comprehensive income (losses) before reclassifications		549	(135)		(111)
Increase (decrease) due to amounts reclassified from AOCI		302	105		86
Change, net of taxes		851	(30)		(138)
Balance, end of year		123	(728)		(698)
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(6,257)	(6,183)		(5,164)
Adjustment to opening balance, net of taxes							0			0
Adjusted balance, beginning of period							(6,183)			(5,164)
Impact of Tax Reform		0	0		(1,020)
Other comprehensive income (losses) before reclassifications		(758)	(240)		(158)
Increase (decrease) due to amounts reclassified from AOCI		206	166		159
Change, net of taxes		(552)	(74)		(1,019)
Balance, end of year		(6,809)	(6,257)		(6,183)
Foreign currency translation adjustment (CTA), net of hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(28,070)	(25,708)		(25,506)
Adjustment to opening balance, net of taxes							0			0
Adjusted balance, beginning of period							(25,708)			(25,506)
Impact of Tax Reform					(1,809)
Other comprehensive income (losses) before reclassifications		(647)	(2,607)		1,607
Increase (decrease) due to amounts reclassified from AOCI		326	245		0
Change, net of taxes		(321)	(2,362)		(202)
Balance, end of year		(28,391)	(28,070)		(25,708)
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(57)	0		0
Adjustment to opening balance, net of taxes							0			0
Adjusted balance, beginning of period							0			0
Impact of Tax Reform					0
Other comprehensive income (losses) before reclassifications		25	(57)		0
Increase (decrease) due to amounts reclassified from AOCI		0	0		0
Change, net of taxes		25	(57)		0
Balance, end of year		(32)	(57)		0
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(37,170)	(34,668)		(32,381)
Adjustment to opening balance, net of taxes			0	[1]	(3)	[1]	(3)	$ 504	[1]	504
Adjusted balance, beginning of period			(37,170)		(34,671)		$ (34,671)	$ (31,877)		$ (31,877)
Impact of Tax Reform					(3,304)
Other comprehensive income (losses) before reclassifications		1,083	(2,824)		726
Increase (decrease) due to amounts reclassified from AOCI		(231)	325		(213)
Change, net of taxes	[2]	852	(2,499)		(2,791)
Balance, end of year		$ (36,318)	$ (37,170)		$ (34,668)

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

[2] Includes the impact of ASU 2018-02, which transferred those amounts from AOCI to 2017 Retained earnings. See Notes 1 and 19 to the Consolidated Financial Statements.