FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Loans	$ 3,824		$ 3,223
Liabilities
Deposits	1,045,607		1,013,170
Allowance for loan losses	12,466	$ 12,329	12,315	$ 12,126	$ 12,354	$ 12,355
Corporate
Assets
Loans	3,804		3,203
Liabilities
Allowance for loan losses	2,353	$ 2,303	2,365	$ 2,330	$ 2,315	$ 2,486
Carrying value
Assets
Investments	74,400		68,900
Securities borrowed and purchased under agreements to resell	81,700		123,000
Loans	670,900		667,100
Other financial assets	281,000		249,700
Liabilities
Deposits	1,043,000		1,011,700
Securities loaned and sold under agreements to repurchase	136,000		133,300
Long-term debt	202,600		193,800
Other financial liabilities	121,800		103,800
Fair value
Assets
Investments	75,300		68,500
Securities borrowed and purchased under agreements to resell	81,700		123,000
Loans	675,600		666,900
Other financial assets	281,400		250,100
Liabilities
Deposits	1,039,800		1,009,500
Securities loaned and sold under agreements to repurchase	136,000		133,300
Long-term debt	212,200		193,700
Other financial liabilities	121,800		103,800
Fair value | Level 1
Assets
Investments	1,900		1,000
Securities borrowed and purchased under agreements to resell	0		0
Loans	0		0
Other financial assets	187,600		172,300
Liabilities
Deposits	0		0
Securities loaned and sold under agreements to repurchase	0		0
Long-term debt	0		0
Other financial liabilities	0		0
Fair value | Level 2
Assets
Investments	71,000		65,400
Securities borrowed and purchased under agreements to resell	81,500		121,600
Loans	9,600		5,600
Other financial assets	15,600		15,800
Liabilities
Deposits	837,500		847,100
Securities loaned and sold under agreements to repurchase	136,000		133,300
Long-term debt	197,800		178,400
Other financial liabilities	20,200		17,200
Fair value | Level 3
Assets
Investments	2,400		2,100
Securities borrowed and purchased under agreements to resell	200		1,400
Loans	666,000		661,300
Other financial assets	78,200		62,000
Liabilities
Deposits	202,300		162,400
Securities loaned and sold under agreements to repurchase	0		0
Long-term debt	14,400		15,300
Other financial liabilities	101,600		86,600
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	8,000		7,800
Lease financing
Liabilities
Lease finance receivables	$ 1,500		$ 1,600