FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets
Loans	$ 4,308		$ 3,486
Liabilities
Deposits	964,038		929,406
Allowance for loan losses	12,366	$ 12,025	12,060	$ 12,439	$ 12,304	$ 12,626
Lease finance receivables	1,800		1,900
Corporate
Assets
Loans	4,281		3,457
Liabilities
Allowance for loan losses	2,474	$ 2,510	2,702	$ 2,766	$ 2,872	$ 2,791
Carrying value
Assets
Investments	58,100		52,100
Federal funds sold and securities borrowed or purchased under agreements to resell	96,300		103,600
Loans	634,700		607,000
Other financial assets	251,200		215,200
Liabilities
Deposits	962,500		928,200
Federal funds purchased and securities loaned or sold under agreements to repurchase	116,000		108,200
Long-term debt	201,800		179,900
Other financial liabilities	128,300		115,300
Fair value
Assets
Investments	58,600		52,000
Federal funds sold and securities borrowed or purchased under agreements to resell	96,300		103,600
Loans	635,800		607,300
Other financial assets	251,700		215,900
Liabilities
Deposits	960,300		927,600
Federal funds purchased and securities loaned or sold under agreements to repurchase	116,000		108,200
Long-term debt	210,500		185,500
Other financial liabilities	128,300		115,300
Fair value | Level 1
Assets
Investments	300		800
Federal funds sold and securities borrowed or purchased under agreements to resell	0		0
Loans	0		0
Other financial assets	7,200		8,200
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	56,300		48,600
Federal funds sold and securities borrowed or purchased under agreements to resell	90,700		98,500
Loans	5,800		7,000
Other financial assets	179,200		153,600
Liabilities
Deposits	819,100		789,700
Federal funds purchased and securities loaned or sold under agreements to repurchase	116,000		107,800
Long-term debt	178,800		156,500
Other financial liabilities	15,400		16,200
Fair value | Level 3
Assets
Investments	2,000		2,600
Federal funds sold and securities borrowed or purchased under agreements to resell	5,600		5,100
Loans	630,000		600,300
Other financial assets	65,300		54,100
Liabilities
Deposits	141,200		137,900
Federal funds purchased and securities loaned or sold under agreements to repurchase	0		400
Long-term debt	31,700		29,000
Other financial liabilities	112,900		99,100
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	$ 2,700		$ 5,200