FAIR VALUE MEASUREMENT - Estimate Fair Value of Financial Instruments Not Carried at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Assets
Loans	$ 6,854	$ 4,085
Liabilities
Deposits	1,280,671	1,070,590
Purchased credit deteriorated	24,956	12,783	$ 12,315	$ 12,355
Lease finance receivables	675,883	699,483
Corporate
Assets
Loans	6,840	4,067
Liabilities
Purchased credit deteriorated	5,402	2,886	$ 2,811	$ 2,943
Lease finance receivables	387,044	389,935
Carrying value
Assets
Investments	110,300	86,400
Securities borrowed and purchased under agreements to resell	109,500	98,100
Loans	643,300	681,200
Other financial assets	383,200	262,400
Liabilities
Deposits	1,278,700	1,068,300
Securities loaned and sold under agreements to repurchase	139,300	125,700
Long-term debt	204,600	193,000
Other financial liabilities	102,400	110,200
Fair value
Assets
Investments	113,200	87,800
Securities borrowed and purchased under agreements to resell	109,500	98,100
Loans	663,900	677,700
Other financial assets	383,200	262,400
Liabilities
Deposits	1,278,800	1,066,700
Securities loaned and sold under agreements to repurchase	139,300	125,700
Long-term debt	221,200	203,800
Other financial liabilities	102,400	110,200
Fair value | Corporate
Liabilities
Lease finance receivables	6,840	4,067
Fair value | Level 1
Assets
Investments	23,300	1,900
Securities borrowed and purchased under agreements to resell	0	0
Loans	0	0
Other financial assets	291,500	177,600
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	87,000	83,800
Securities borrowed and purchased under agreements to resell	109,500	98,100
Loans	600	4,700
Other financial assets	18,100	16,300
Liabilities
Deposits	1,093,300	875,500
Securities loaned and sold under agreements to repurchase	139,300	125,700
Long-term debt	197,800	187,300
Other financial liabilities	19,200	37,500
Fair value | Level 3
Assets
Investments	2,900	2,100
Securities borrowed and purchased under agreements to resell	0	0
Loans	663,300	673,000
Other financial assets	73,600	68,500
Liabilities
Deposits	185,500	191,200
Securities loaned and sold under agreements to repurchase	0	0
Long-term debt	23,400	16,500
Other financial liabilities	83,200	72,700
Fair value | Level 3 | Corporate
Fair value measurements additional disclosures
Unfunded lending commitments	7,300	5,100
Lease financing
Liabilities
Lease finance receivables	700	1,400
Lease financing | Corporate
Liabilities
Lease finance receivables	$ 738	$ 1,385