DERIVATIVES - Fair Value Hedges (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Other revenue
Gain (loss) on fair value hedges
Gain (loss) on the hedging derivatives included in assessment of the effectiveness of fair value hedges	$ 1,278	$ 304	$ (2,187)
Gain (loss) on the hedged item in designated and qualifying fair value hedges	(1,278)	(304)	2,188
Net gain (loss) on the hedging derivatives excluded from assessment of the effectiveness of fair value hedges	58	(68)	(23)
Net interest revenue
Gain (loss) on fair value hedges
Gain (loss) on the hedging derivatives included in assessment of the effectiveness of fair value hedges	4,189	2,273	794
Gain (loss) on the hedged item in designated and qualifying fair value hedges	(4,537)	(2,085)	(747)
Net gain (loss) on the hedging derivatives excluded from assessment of the effectiveness of fair value hedges	(23)	3	(5)
Interest rate contracts | Other revenue
Gain (loss) on fair value hedges
Gain (loss) on the hedging derivatives included in assessment of the effectiveness of fair value hedges	0	0	0
Gain (loss) on the hedged item in designated and qualifying fair value hedges	0	0	0
Net gain (loss) on the hedging derivatives excluded from assessment of the effectiveness of fair value hedges	0	0	0
Interest rate contracts | Net interest revenue
Gain (loss) on fair value hedges
Gain (loss) on the hedging derivatives included in assessment of the effectiveness of fair value hedges	4,189	2,273	794
Gain (loss) on the hedged item in designated and qualifying fair value hedges	(4,537)	(2,085)	(747)
Net gain (loss) on the hedging derivatives excluded from assessment of the effectiveness of fair value hedges	(23)	3	(5)
Foreign exchange contracts
Gain (loss) on fair value hedges
Cross-currency basis included in accumulated other comprehensive income	(23)	33
Foreign exchange contracts | Other revenue
Gain (loss) on fair value hedges
Gain (loss) on the hedging derivatives included in assessment of the effectiveness of fair value hedges	1,442	337	(2,064)
Gain (loss) on the hedged item in designated and qualifying fair value hedges	(1,442)	(337)	2,064
Net gain (loss) on the hedging derivatives excluded from assessment of the effectiveness of fair value hedges	(73)	(109)	(4)
Foreign exchange contracts | Net interest revenue
Gain (loss) on fair value hedges
Gain (loss) on the hedging derivatives included in assessment of the effectiveness of fair value hedges	0	0	0
Gain (loss) on the hedged item in designated and qualifying fair value hedges	0	0	0
Net gain (loss) on the hedging derivatives excluded from assessment of the effectiveness of fair value hedges	0	0	0
Commodity and other contract options | Other revenue
Gain (loss) on fair value hedges
Gain (loss) on the hedging derivatives included in assessment of the effectiveness of fair value hedges	(164)	(33)	(123)
Gain (loss) on the hedged item in designated and qualifying fair value hedges	164	33	124
Net gain (loss) on the hedging derivatives excluded from assessment of the effectiveness of fair value hedges	131	41	(19)
Commodity and other contract options | Net interest revenue
Gain (loss) on fair value hedges
Gain (loss) on the hedging derivatives included in assessment of the effectiveness of fair value hedges	0	0	0
Gain (loss) on the hedged item in designated and qualifying fair value hedges	0	0	0
Net gain (loss) on the hedging derivatives excluded from assessment of the effectiveness of fair value hedges	$ 0	$ 0	$ 0