FAIR VALUE MEASUREMENT - Level 3 Fair Value Rollforward Narrative (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Loans
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Assets (liabilities) transferred out of Level 3			$ 2,000
Long-term debt
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Assets (liabilities) transferred out of Level 3	$ (1,100)		(7,100)
Transfers into Level 3, liabilities	2,052	$ 1,396	6,921	$ 6,459
Long-term debt | Option Volatility
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers into Level 3, liabilities			5,900
Long-term debt | Equity Volatility
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers into Level 3, liabilities			900
Trading account assets and liabilities
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Assets (liabilities) transferred out of Level 3	(924)	33	(1,459)	216
Transfers into Level 3, derivative asset	(499)	(66)	(891)	1,332
Trading account assets and liabilities | Interest rate contracts
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Assets (liabilities) transferred out of Level 3	(56)	7	(377)	9
Transfers into Level 3, derivative asset	(79)	(17)	94	1,597
Trading account assets and liabilities | Equity contracts
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Assets (liabilities) transferred out of Level 3	(668)	69	(1,566)	(167)
Transfers into Level 3, liabilities			1,000
Transfers into Level 3, derivative asset	$ (370)	$ (41)	$ (968)	$ (432)