FAIR VALUE MEASUREMENT - Level 3 Fair Value Rollforward Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 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	$ 2,100		$ 2,100
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	(6,000)	$ 1,500	(6,000)	$ 4,200
Transfers into Level 3, liabilities	3,937	1,875	4,869	5,063
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			4,000
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			800
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	(531)	52	(535)	183
Transfers into Level 3, derivative asset	(616)	135	(392)	1,398
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	(321)	(20)	(321)	2
Transfers into Level 3, derivative asset	$ 218	$ 231	$ 173	$ 1,614