CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2016	Dec. 31, 2014
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
AOCI, Pretax		$ (42,035)	$ (38,440)	$ (31,060)
AOCI, Tax Effect		9,654	9,096	7,844
Balance, beginning of year		226,143	223,092
Adjustment to opening balance, Pretax			803	(26)
Adjusted balance, Pretax			(41,232)	(38,466)
Adjustment to opening balance, Tax Effect			(299)	11
Adjusted Balance, Tax Effect			9,355	9,107
OCI, Net of Tax [Abstract]
Pretax		4	(3,569)	(7,380)
Tax Effect		(509)	(547)	(1,252)
Change, net of taxes		(2,791)	(3,022)	(6,128)
Impact of Tax Reform related to AOCI reclassification		(3,304)
AOCI, Tax Effect		9,864	9,654	9,096
AOCI, Pretax		(41,228)	(42,035)	(38,440)
Balance, end of year		201,672	226,143	223,092
Change in accumulated other comprehensive income (loss)
Increase (decrease) to AOCI		(34,668)	(32,381)
Net unrealized gains (losses) on investment securities
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to opening balance, net of taxes			504	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance			(295)	(907)
OCI, Net of Tax [Abstract]
Pretax		(1,088)	167	(1,462)
Tax Effect		(448)	59	(498)
Change, net of taxes		(863)	108	(964)
Impact of Tax Reform related to AOCI reclassification		(223)
Change in accumulated other comprehensive income (loss)
Increase (decrease) to AOCI		(1,158)	(799)	(907)		$ 57
Debt valuation adjustment (DVA)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to opening balance, net of taxes			0	(15)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance			(352)	(15)
OCI, Net of Tax [Abstract]
Pretax		(680)	(538)
Tax Effect		(250)	(201)
Change, net of taxes		(569)	(337)	0
Impact of Tax Reform related to AOCI reclassification		(139)
Change in accumulated other comprehensive income (loss)
Increase (decrease) to AOCI		(921)	(352)	0		0
Cash flow hedges
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to opening balance, net of taxes			0	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance			(560)	(617)
OCI, Net of Tax [Abstract]
Pretax		(37)	84	468
Tax Effect		(12)	27	176
Change, net of taxes		(138)	57	292
Impact of Tax Reform related to AOCI reclassification		(113)
Change in accumulated other comprehensive income (loss)
Increase (decrease) to AOCI		(698)	(560)	(617)		(909)
Benefit plans
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of year		(5,164)	(5,116)	(5,159)
Adjustment to opening balance, net of taxes			0	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance			(5,164)	(5,116)
OCI, Net of Tax [Abstract]
Pretax		14	(78)	19
Tax Effect		13	(30)	(24)
Change, net of taxes		(1,019)	(48)	43
Impact of Tax Reform related to AOCI reclassification		(1,020)	0	0
Balance, end of year		(6,183)	(5,164)	(5,116)
Change in accumulated other comprehensive income (loss)
Increase (decrease) to AOCI		(6,183)	(5,164)	(5,116)		(5,159)
Foreign currency translation adjustment
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to opening balance, net of taxes			0	0
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance			(25,506)	(22,704)
OCI, Net of Tax [Abstract]
Pretax		1,795	(3,204)	(6,405)
Tax Effect		188	(402)	(906)
Change, net of taxes		(202)	(2,802)	(5,499)
Impact of Tax Reform related to AOCI reclassification		(1,809)
Change in accumulated other comprehensive income (loss)
Increase (decrease) to AOCI		(25,708)	(25,506)	(22,704)		(17,205)
Accumulated other comprehensive income (loss)
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Balance, beginning of year		(32,381)	(29,344)	(23,216)
Adjustment to opening balance, net of taxes	[1]		504	(15)
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest, Adjusted Balance			(31,877)	(29,359)		(23,216)
OCI, Net of Tax [Abstract]
Change, net of taxes	[2]	(2,791)	(3,022)	(6,128)
Balance, end of year		(34,668)	(32,381)	(29,344)
Change in accumulated other comprehensive income (loss)
Increase (decrease) to AOCI		(34,668)	$ (32,381)	$ (29,344)		$ (23,216)
Accounting Standards Update 2018-02
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]
Adjustment to opening balance, net of taxes		(3,304)
OCI, Net of Tax [Abstract]
Impact of Tax Reform related to AOCI reclassification		(3,304)
Accounting Standards Update 2018-02 | Net unrealized gains (losses) on investment securities
OCI, Net of Tax [Abstract]
Impact of Tax Reform related to AOCI reclassification		(223)
Accounting Standards Update 2018-02 | Debt valuation adjustment (DVA)
OCI, Net of Tax [Abstract]
Impact of Tax Reform related to AOCI reclassification		(139)
Accounting Standards Update 2018-02 | Cash flow hedges
OCI, Net of Tax [Abstract]
Impact of Tax Reform related to AOCI reclassification		(113)
Accounting Standards Update 2018-02 | Benefit plans
OCI, Net of Tax [Abstract]
Impact of Tax Reform related to AOCI reclassification		(1,020)
Accounting Standards Update 2018-02 | Foreign currency translation adjustment
OCI, Net of Tax [Abstract]
Impact of Tax Reform related to AOCI reclassification		$ (1,809)
Accounting Standards Update 2016-01
Change in accumulated other comprehensive income (loss)
Increase (decrease) to AOCI					$ (15)

[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.