FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets
Loans	$ 3,000		$ 4,374
Liabilities
Deposits	996,730		959,822
Allowance for loan losses	12,126	$ 12,354	12,355	$ 12,025	$ 12,030	$ 12,060
Lease finance receivables	1,600		1,700
Corporate
Assets
Loans	2,978		4,349
Liabilities
Allowance for loan losses	2,330	$ 2,315	2,486	$ 2,510	$ 2,535	$ 2,702
Carrying value
Assets
Investments	58,800		60,200
Federal funds sold and securities borrowed or purchased under agreements to resell	96,400		99,500
Loans	654,400		648,600
Other financial assets	265,000		242,600
Liabilities
Deposits	995,100		958,400
Federal funds purchased and securities loaned or sold under agreements to repurchase	128,600		115,600
Long-term debt	201,400		205,300
Other financial liabilities	111,900		129,900
Fair value
Assets
Investments	58,300		60,600
Federal funds sold and securities borrowed or purchased under agreements to resell	96,400		99,500
Loans	648,800		644,900
Other financial assets	265,400		243,000
Liabilities
Deposits	992,800		955,600
Federal funds purchased and securities loaned or sold under agreements to repurchase	128,600		115,600
Long-term debt	204,600		214,000
Other financial liabilities	111,900		129,900
Fair value | Level 1
Assets
Investments	1,100		500
Federal funds sold and securities borrowed or purchased under agreements to resell	0		0
Loans	0		0
Other financial assets	187,300		166,400
Liabilities
Deposits	3,000		0
Federal funds purchased and securities loaned or sold under agreements to repurchase	0		0
Long-term debt	0		0
Other financial liabilities	0		0
Fair value | Level 2
Assets
Investments	55,200		57,500
Federal funds sold and securities borrowed or purchased under agreements to resell	91,800		94,400
Loans	5,600		6,000
Other financial assets	13,600		14,100
Liabilities
Deposits	844,400		816,100
Federal funds purchased and securities loaned or sold under agreements to repurchase	128,500		115,600
Long-term debt	186,300		187,200
Other financial liabilities	15,700		15,500
Fair value | Level 3
Assets
Investments	2,000		2,600
Federal funds sold and securities borrowed or purchased under agreements to resell	4,600		5,100
Loans	643,200		638,900
Other financial assets	64,500		62,500
Liabilities
Deposits	145,400		139,500
Federal funds purchased and securities loaned or sold under agreements to repurchase	100		0
Long-term debt	18,300		26,800
Other financial liabilities	96,200		114,400
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	$ 4,200		$ 3,200