FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Loans	$ 3,223	$ 4,374
Liabilities
Deposits	1,013,170	959,822
Allowance for loan losses	12,315	12,355	$ 12,060	$ 12,626
Lease finance receivables	1,600	1,700
Corporate
Assets
Loans	3,203	4,349
Carrying value
Assets
Investments	68,900	60,200
Federal funds sold and securities borrowed and purchased under agreements to resell	123,000	99,500
Loans	667,100	648,600
Other financial assets	249,700	242,600
Liabilities
Deposits	1,011,700	958,400
Federal funds purchased and securities loaned and sold under agreements to repurchase	133,300	115,600
Long-term debt	193,800	205,300
Other financial liabilities	103,800	129,900
Fair value
Assets
Investments	68,500	60,600
Federal funds sold and securities borrowed and purchased under agreements to resell	123,000	99,500
Loans	666,900	644,900
Other financial assets	250,100	243,000
Liabilities
Deposits	1,009,500	955,600
Federal funds purchased and securities loaned and sold under agreements to repurchase	133,300	115,600
Long-term debt	193,700	214,000
Other financial liabilities	103,800	129,900
Fair value | Level 1
Assets
Investments	1,000	500
Federal funds sold and securities borrowed and purchased under agreements to resell	0	0
Loans	0	0
Other financial assets	172,300	166,400
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	65,400	57,500
Federal funds sold and securities borrowed and purchased under agreements to resell	121,600	94,400
Loans	5,600	6,000
Other financial assets	15,800	14,100
Liabilities
Deposits	847,100	816,100
Federal funds purchased and securities loaned and sold under agreements to repurchase	133,300	115,600
Long-term debt	178,400	187,200
Other financial liabilities	17,200	15,500
Fair value | Level 3
Assets
Investments	2,100	2,600
Federal funds sold and securities borrowed and purchased under agreements to resell	1,400	5,100
Loans	661,300	638,900
Other financial assets	62,000	62,500
Liabilities
Deposits	162,400	139,500
Federal funds purchased and securities loaned and sold under agreements to repurchase	0	0
Long-term debt	15,300	26,800
Other financial liabilities	86,600	114,400
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	$ 7,800	$ 3,200