SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Accounting Changes (Details) - USD ($) $ in Millions			3 Months Ended											12 Months Ended
		Jan. 01, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017		Jan. 01, 2019	Jan. 01, 2018	Jan. 01, 2017	Dec. 31, 2016		Jan. 01, 2016
New Accounting Pronouncements or Change in Accounting Principle
Operating lease liability			$ 3,340				$ 3,300							$ 3,340	$ 3,300
Operating lease, right-of-use asset			3,100											3,100
Impact of non-cash charge related to Tax Reform												$ 22,600					$ 22,594
Tax Reform benefit							94								94
Benefit relating to reduction in valuation allowance against FTC carry-forwards and U.S. residual DTAs							1,200
Charge for quasi-territorial tax system							200
Charge related to impact of deemed repatriation of undistributed earnings							900
Revenues, net of interest expense			18,378	$ 18,574	$ 18,758	$ 18,576	17,124		$ 18,389	$ 18,469	$ 18,872			74,286	72,854		72,444
Operating expenses			10,454	$ 10,464	$ 10,500	$ 10,584	9,893		$ 10,311	$ 10,712	$ 10,925			42,002	41,841		42,232
Increase (decrease) in retained earnings			165,369				151,347							165,369	151,347
Fair value			280,265				288,038							280,265	288,038
Net cash used in investing activities of continuing operations														(23,374)	(73,118)		(38,751)
Stockholders’ equity			(193,946)				(197,074)					(201,672)		(193,946)	(197,074)		(201,672)
Net deferred tax assets			23,094				22,903							23,094	22,903
Increase (decrease) in allowance for loan losses			12,783				12,315					12,355		12,783	12,315		12,355					$ 12,060
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle
Operating lease liability																			$ 4,400
Operating lease, right-of-use asset																			4,400
Accounting Standards Update 2016-16
New Accounting Pronouncements or Change in Accounting Principle
Increase in deferred income tax assets																				$ 300
Increase (decrease) in retained earnings																				(80)
Prepaid tax																				380
Accounting Standards Update 2017-12
New Accounting Pronouncements or Change in Accounting Principle
Fair value																				$ 4,000
Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle
Increase (decrease) in retained earnings												3,300					3,300
Accounting Standards Update 2017-08 | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle
Stockholders’ equity																					$ 156
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle
Net cash used in investing activities of continuing operations																	19,300
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect of new accounting principle	[1]						151					(84)			151		(84)					(660)
Stockholders’ equity			(165,369)				(151,347)					(138,425)		(165,369)	(151,347)		(138,425)					(146,477)
Retained earnings | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect of new accounting principle																			$ 151
Retained earnings | Accounting Standards Update 2017-08 | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect of new accounting principle																					(660)
Accumulated other comprehensive income (loss)
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect of new accounting principle							0	[1]				(3)	[1]		0	[1]	(3)	[1]			(3)	504	[1]	$ 504
Stockholders’ equity			$ 36,318				$ 37,170					$ 34,668		36,318	37,170		$ 34,668					$ 32,381
Accumulated other comprehensive income (loss) | Accounting Standards Update 2017-08 | New Accounting Pronouncement, Early Adoption, Effect
New Accounting Pronouncements or Change in Accounting Principle
Cumulative effect of new accounting principle																					$ 504
Restatement | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle
Revenues, net of interest expense														1,000	1,000
Operating expenses														$ 1,000	$ 1,000
Forecast | Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle
Increase (decrease) in retained earnings		$ (3,100)
Net deferred tax assets		1,000
Increase (decrease) in allowance for loan losses		$ 4,100
Pretax percentage increase allowance for credit losses		29.00%
Build to allowance for credit losses		$ 4,900
Loss coverage period		23 months
Releases from allowance for credit losses		$ (800)
Forecast | Accounting Guidance Prior to Adoption of ASU 2016-13
New Accounting Pronouncements or Change in Accounting Principle
Loss coverage period		14 months

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