CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Change in Each Component of AOCI (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016		Jan. 01, 2018	Jan. 01, 2017	Jan. 01, 2016	Dec. 31, 2015
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		$ 201,672	$ 226,143
Change, net of taxes		(2,499)	(2,791)		$ (3,022)
Balance, end of year		197,074	201,672		226,143
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(1,158)	(799)		(907)
Adjustment to opening balance, net of taxes							$ (3)	$ 504	$ 0
Adjusted balance, beginning of period							(1,161)	(295)	(907)
Impact of Tax Reform			(223)
Other comprehensive income (losses) before reclassifications		(866)	(186)		531
Increase (decrease) due to amounts reclassified from AOCI		(223)	(454)		(423)
Change, net of taxes		(1,089)	(863)		108
Balance, end of year		(2,250)	(1,158)		(799)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(921)	(352)		0
Adjustment to opening balance, net of taxes							0	0	(15)
Adjusted balance, beginning of period							(921)	(352)	(15)
Impact of Tax Reform			(139)
Other comprehensive income (losses) before reclassifications		1,081	(426)		(335)
Increase (decrease) due to amounts reclassified from AOCI		32	(4)		(2)
Change, net of taxes		1,113	(569)		(337)
Balance, end of year		192	(921)		(352)
Cash flow hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(698)	(560)		(617)
Adjustment to opening balance, net of taxes							0	0	0
Adjusted balance, beginning of period							(698)	(560)	(617)
Impact of Tax Reform			(113)
Other comprehensive income (losses) before reclassifications		(135)	(111)		(88)
Increase (decrease) due to amounts reclassified from AOCI		105	86		145
Change, net of taxes		(30)	(138)		57
Balance, end of year		(728)	(698)		(560)
Benefit plans
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(6,183)	(5,164)		(5,116)
Adjustment to opening balance, net of taxes							0	0	0
Adjusted balance, beginning of period							(6,183)	(5,164)	(5,116)
Impact of Tax Reform		0	(1,020)		0
Other comprehensive income (losses) before reclassifications		(240)	(158)		(208)
Increase (decrease) due to amounts reclassified from AOCI		166	159		160
Change, net of taxes		(74)	(1,019)		(48)
Balance, end of year		(6,257)	(6,183)		(5,164)
Foreign currency translation adjustment (CTA), net of hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(25,708)	(25,506)		(22,704)
Adjustment to opening balance, net of taxes							0	0	0
Adjusted balance, beginning of period							(25,708)	(25,506)	(22,704)
Impact of Tax Reform			(1,809)
Other comprehensive income (losses) before reclassifications		(2,607)	1,607		(2,802)
Increase (decrease) due to amounts reclassified from AOCI		245	0		0
Change, net of taxes		(2,362)	(202)		(2,802)
Balance, end of year		(28,070)	(25,708)		(25,506)
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		0	0		0
Adjustment to opening balance, net of taxes							0	0	0
Adjusted balance, beginning of period							0	0	0
Impact of Tax Reform			0
Other comprehensive income (losses) before reclassifications		(57)	0		0
Increase (decrease) due to amounts reclassified from AOCI		0	0		0
Change, net of taxes		(57)	0		0
Balance, end of year		(57)	0		0
Accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss)
Balance, beginning of year		(34,668)	(32,381)		(29,344)
Adjustment to opening balance, net of taxes			(3)	[1]	504	[1]	(3)	504	(15)	$ (15)	[1]
Adjusted balance, beginning of period			(34,671)		(31,877)		$ (34,671)	$ (31,877)	$ (29,359)	$ (29,359)
Impact of Tax Reform			(3,304)
Other comprehensive income (losses) before reclassifications		(2,824)	726		(2,902)
Increase (decrease) due to amounts reclassified from AOCI		325	(213)		(120)
Change, net of taxes	[2]	(2,499)	(2,791)		(3,022)
Balance, end of year		$ (37,170)	$ (34,668)		$ (32,381)

[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 Retained earnings. See Notes 1 and 19 to the Consolidated Financial Statements.