LOANS - Corporate Loans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Loans
Loans, net of unearned income	$ 667,034	$ 624,369
Corporate
Loans
Total loans	334,141	300,010
Net unearned income (loss)	(763)	(704)
Loans, net of unearned income	333,378	299,306
Loans sold and/or reclassified to held-for-sale	1,000	4,200
Corporate | Commercial and industrial
Loans
Loans, net of unearned income	141,322	129,116
Corporate | Financial institutions
Loans
Loans, net of unearned income	73,757	61,560
Corporate | Mortgage and real estate
Loans
Loans, net of unearned income	51,964	44,031
Corporate | Lease financing
Loans
Loans, net of unearned income	1,655	1,769
In U.S. offices | Corporate
Loans
Total loans	169,781	159,813
In U.S. offices | Corporate | Commercial and industrial
Loans
Total loans	51,319	49,586
In U.S. offices | Corporate | Financial institutions
Loans
Total loans	39,128	35,517
In U.S. offices | Corporate | Mortgage and real estate
Loans
Total loans	44,683	38,691
In U.S. offices | Corporate | Installment, revolving credit and other
Loans
Total loans	33,181	34,501
In U.S. offices | Corporate | Lease financing
Loans
Total loans	1,470	1,518
In offices outside the U.S. | Corporate
Loans
Total loans	164,360	140,197
In offices outside the U.S. | Corporate | Commercial and industrial
Loans
Total loans	93,750	81,882
In offices outside the U.S. | Corporate | Financial institutions
Loans
Total loans	35,273	26,886
In offices outside the U.S. | Corporate | Mortgage and real estate
Loans
Total loans	7,309	5,363
In offices outside the U.S. | Corporate | Installment, revolving credit and other
Loans
Total loans	22,638	19,965
In offices outside the U.S. | Corporate | Lease financing
Loans
Total loans	190	251
In offices outside the U.S. | Corporate | Government and official institutions
Loans
Total loans	$ 5,200	$ 5,850