BASIS OF PRESENTATION, UPDATED ACCOUNTING POLICIES AND ACCOUNTING CHANGES - Accounting Changes (Details) - USD ($) $ in Millions		3 Months Ended				6 Months Ended
	Jan. 01, 2020	Jun. 30, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		$ 19,238		$ 26,298	$ 20,380	$ 19,238	$ 26,298	$ 21,638	$ 24,956	$ 12,783
Pretax percentage increase allowance for credit losses	29.00%
Increase (decrease) in retained earnings		179,686				179,686			168,272
Build to allowance for credit losses		(2,184)		5,195		(5,252)	9,289
Decrease in other assets		(116,089)				(116,089)			(110,683)
Increase (decrease) in provisions for credit losses on loans		(1,126)		7,990		(2,605)	14,367
Increase in operating expenses		$ 11,192		10,460		$ 22,265	$ 21,103
Variable post-charge-off third-party collection costs
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ (443)				0			0	0	(443)
Increase (decrease) in retained earnings	330
Decrease in other assets	113
Increase (decrease) in provisions for credit losses on loans			$ 122	339	(18)
Increase in operating expenses				$ 45	49
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	4,100				$ 0			$ 0	$ 0	$ 4,201
Increase (decrease) in retained earnings	(3,100)
Increase in deferred tax assets	1,000
Build to allowance for credit losses	4,900
Release of reserves	$ 800
Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loss coverage period	23 months
Accounting Guidance Prior to Adoption of Accounting Standards Update 2016-13
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Loss coverage period	14 months