FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets
Loans	$ 4,085	$ 3,223
Liabilities
Deposits	1,070,590	1,013,170
Allowance for loan losses	12,783	12,315	$ 12,355	$ 12,060
Corporate
Assets
Loans	4,067	3,203
Carrying value
Assets
Investments	86,400	68,900
Securities borrowed and purchased under agreements to resell	98,100	123,000
Loans	681,200	667,100
Other financial assets	262,400	249,700
Liabilities
Deposits	1,068,300	1,011,700
Securities loaned and sold under agreements to repurchase	125,700	133,300
Long-term debt	193,000	193,800
Other financial liabilities	110,200	103,800
Fair value
Assets
Investments	87,800	68,500
Securities borrowed and purchased under agreements to resell	98,100	123,000
Loans	677,700	666,900
Other financial assets	262,400	250,100
Liabilities
Deposits	1,066,700	1,009,500
Securities loaned and sold under agreements to repurchase	125,700	133,300
Long-term debt	203,800	193,700
Other financial liabilities	110,200	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	177,600	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	83,800	65,400
Securities borrowed and purchased under agreements to resell	98,100	121,600
Loans	4,700	5,600
Other financial assets	16,300	15,800
Liabilities
Deposits	875,500	847,100
Securities loaned and sold under agreements to repurchase	125,700	133,300
Long-term debt	187,300	178,400
Other financial liabilities	37,500	17,200
Fair value | Level 3
Assets
Investments	2,100	2,100
Securities borrowed and purchased under agreements to resell	0	1,400
Loans	673,000	661,300
Other financial assets	68,500	62,000
Liabilities
Deposits	191,200	162,400
Securities loaned and sold under agreements to repurchase	0	0
Long-term debt	16,500	15,300
Other financial liabilities	72,700	86,600
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	5,100	7,800
Lease financing
Liabilities
Lease finance receivables	$ 1,400	$ 1,600