FAIR VALUE MEASUREMENT - Level 3 Fair Value Rollforward Narrative (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Long-term debt
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers into Level 3, liabilities	$ 932	$ 3,189
Transfers out of Level 3, liabilities	2	2,693
Interest rate contracts | Long-term debt
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers into Level 3, liabilities		2,000
Trading account assets and liabilities
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers into Level 3, derivative asset	224	1,263
Trading account assets and liabilities | Interest rate contracts
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers into Level 3, derivative asset	$ (45)	1,383
Securities sold, not yet purchased | Long-term debt
Fair value, assets and liabilities measured on recurring basis, level 3 fair-value category disclosures
Transfers into Level 3, liabilities		$ 1,200