FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets
Loans	$ 4,239		$ 4,374
Liabilities
Deposits	1,005,176		959,822
Allowance for loan losses	12,336	$ 12,126	12,355	$ 12,366	$ 12,025	$ 12,060
Lease finance receivables	1,600		1,700
Corporate
Assets
Loans	4,218		4,349
Liabilities
Allowance for loan losses	2,339	$ 2,330	2,486	$ 2,474	$ 2,510	$ 2,702
Carrying value
Assets
Investments	58,900		60,200
Federal funds sold and securities borrowed and purchased under agreements to resell	102,500		99,500
Loans	656,700		648,600
Other financial assets	263,900		242,600
Liabilities
Deposits	1,003,700		958,400
Federal funds purchased and securities loaned or sold under agreements to repurchase	127,800		115,600
Long-term debt	198,500		205,300
Other financial liabilities	110,600		129,900
Fair value
Assets
Investments	58,000		60,600
Federal funds sold and securities borrowed and purchased under agreements to resell	102,500		99,500
Loans	655,200		644,900
Other financial assets	264,400		243,000
Liabilities
Deposits	1,002,800		955,600
Federal funds purchased and securities loaned or sold under agreements to repurchase	127,800		115,600
Long-term debt	200,600		214,000
Other financial liabilities	110,600		129,900
Fair value | Level 1
Assets
Investments	1,100		500
Federal funds sold and securities borrowed and purchased under agreements to resell	0		0
Loans	0		0
Other financial assets	184,600		166,400
Liabilities
Deposits	0		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	54,900		57,500
Federal funds sold and securities borrowed and purchased under agreements to resell	100,500		94,400
Loans	5,200		6,000
Other financial assets	14,700		14,100
Liabilities
Deposits	836,700		816,100
Federal funds purchased and securities loaned or sold under agreements to repurchase	127,800		115,600
Long-term debt	186,300		187,200
Other financial liabilities	16,100		15,500
Fair value | Level 3
Assets
Investments	2,000		2,600
Federal funds sold and securities borrowed and purchased under agreements to resell	2,000		5,100
Loans	650,000		638,900
Other financial assets	65,100		62,500
Liabilities
Deposits	166,100		139,500
Federal funds purchased and securities loaned or sold under agreements to repurchase	0		0
Long-term debt	14,300		26,800
Other financial liabilities	94,500		114,400
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	$ 3,200		$ 3,200