DERIVATIVES - Cash Flow Hedges (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) recognized in AOCI on derivative	$ (452)	$ 2,486
Net pretax change in cash flow hedges included within AOCI	(729)	2,484
Cash flow hedge gain expected to be reclassified from AOCI within 12 months	$ 1,100
Maximum length of time hedged in cash flow hedge	10 years
Interest rate contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) recognized in AOCI on derivative	$ (455)	2,497
Foreign exchange contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) recognized in AOCI on derivative	3	(11)
Other revenue
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	(1)	(1)
Other revenue | Interest rate contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	0	0
Other revenue | Foreign exchange contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	(1)	(1)
Net interest revenue
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	278	3
Net interest revenue | Interest rate contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	278	3
Net interest revenue | Foreign exchange contracts
Pretax change in accumulated other comprehensive income (loss)
Amount of gain (loss) reclassified from AOCI to earnings	$ 0	$ 0