BASIS OF PRESENTATION, UPDATED ACCOUNTING POLICIES AND ACCOUNTING CHANGES - Accounting Changes (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2020	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses		$ 21,638			$ 20,380	$ 24,956	$ 12,783
Pretax percentage increase allowance for credit losses	29.00%
Increase (decrease) in retained earnings		174,816				168,272
Build to allowance for credit losses		(3,068)			4,094
Decrease in other assets		(109,433)				(110,683)
Increase (decrease) in provisions for credit losses on loans		(1,479)			6,377
Increase in operating expenses		$ 11,073			10,643
Revision of Prior Period, Change in Accounting Principle, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	$ (443)					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	$ 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