FAIR VALUE MEASUREMENT - Level 3 Roll Forward Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Federal funds sold and securities borrowed or purchased under agreements to resell
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	$ 1,198	$ 28
Transfers into Level 3, assets	0	0
Trading non-derivative assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	3,737	9,045
Transfers into Level 3, assets	1,321	3,835
Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	1,021	2,054
Transfers into Level 3, assets	186	954
Loans
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	16	1,074
Transfers into Level 3, assets	80	89
Other debt securities | Trading non-derivative assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	2,691	2,708
Transfers into Level 3, assets	474	2,070
Other debt securities | Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	0	0
Transfers into Level 3, assets	0	10
Mortgage-backed securities - U.S. agency-sponsored | Trading non-derivative assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	174	1,087
Transfers into Level 3, assets	176	510
Mortgage-backed securities - U.S. agency-sponsored | Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	94	72
Transfers into Level 3, assets	1	25
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets		4,000
Equity securities | Trading non-derivative assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	58	4,057
Transfers into Level 3, assets	210	279
Equity securities | Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	0	5
Transfers into Level 3, assets	0	5
Non-marketable equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets		3,200
Non-marketable equity securities | Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	0	32
Transfers into Level 3, assets	2	336
Hedging derivatives
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets		500
State and municipal securities | Trading non-derivative assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	48	256
Transfers into Level 3, assets	24	195
State and municipal securities | Investments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers out of Level 3, assets	517	1,598
Transfers into Level 3, assets	70	467
Long-term debt
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers into Level 3, liabilities	1,251	3,792
Transfers out of Level 3, liabilities	1,836	4,350
Securities sold, not yet purchased | Trading account liabilities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Transfers into Level 3, liabilities	22	1,185
Transfers out of Level 3, liabilities	$ 796	$ 109