FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Loans	$ 3,874	$ 3,223
Liabilities
Deposits	1,030,355	1,013,170
Allowance for loan losses	12,329	12,315	$ 12,354	$ 12,355
Lease finance receivables	1,500	1,600
Corporate
Assets
Loans	3,854	3,203
Liabilities
Allowance for loan losses	2,303	2,365	$ 2,315	$ 2,486
Carrying value
Assets
Investments	72,500	68,900
Federal funds sold and securities borrowed and purchased under agreements to resell	102,400	123,000
Loans	664,600	667,100
Other financial assets	276,500	249,700
Liabilities
Deposits	1,028,000	1,011,700
Federal funds purchased and securities loaned and sold under agreements to repurchase	144,100	133,300
Long-term debt	199,400	193,800
Other financial liabilities	111,200	103,800
Fair value
Assets
Investments	72,700	68,500
Federal funds sold and securities borrowed and purchased under agreements to resell	102,400	123,000
Loans	669,300	666,900
Other financial assets	277,000	250,100
Liabilities
Deposits	1,026,200	1,009,500
Federal funds purchased and securities loaned and sold under agreements to repurchase	144,100	133,300
Long-term debt	204,000	193,700
Other financial liabilities	111,200	103,800
Fair value | Level 1
Assets
Investments	1,000	1,000
Federal funds sold and securities borrowed and purchased under agreements to resell	0	0
Loans	0	0
Other financial assets	189,400	172,300
Liabilities
Deposits	0	0
Federal funds purchased and 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	69,600	65,400
Federal funds sold and securities borrowed and purchased under agreements to resell	102,200	121,600
Loans	11,000	5,600
Other financial assets	16,500	15,800
Liabilities
Deposits	824,000	847,100
Federal funds purchased and securities loaned and sold under agreements to repurchase	144,100	133,300
Long-term debt	188,000	178,400
Other financial liabilities	18,900	17,200
Fair value | Level 3
Assets
Investments	2,100	2,100
Federal funds sold and securities borrowed and purchased under agreements to resell	200	1,400
Loans	658,300	661,300
Other financial assets	71,100	62,000
Liabilities
Deposits	202,200	162,400
Federal funds purchased and securities loaned and sold under agreements to repurchase	0	0
Long-term debt	16,000	15,300
Other financial liabilities	92,300	86,600
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	$ 7,400	$ 7,800