LOANS - Corporate Loans (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Loans
Total loans, net of unearned income	$ 676,834		$ 676,834		$ 675,883
Lease financing
Loans
Total loans, net of unearned income	600		600		700
Corporate
Loans
Total loans, net of unearned income	400,939		400,939		387,044
Unearned income	(841)		(841)		(844)
Loans sold and/or reclassified to held-for-sale	1,700	$ 800	3,100	$ 1,000
Corporate | Commercial and industrial
Loans
Total loans, net of unearned income	152,134		152,134		156,340
Corporate | Financial institutions
Loans
Total loans, net of unearned income	100,989		100,989		87,689
Corporate | Mortgage and real estate
Loans
Total loans, net of unearned income	73,156		73,156		72,038
Corporate | Lease financing
Loans
Total loans, net of unearned income	602		602		738
Corporate | In North America offices
Loans
Total loans, net of unearned income	208,509		208,509		201,632
Corporate | In North America offices | Commercial and industrial
Loans
Total loans, net of unearned income	53,549		53,549		57,731
Corporate | In North America offices | Financial institutions
Loans
Total loans, net of unearned income	65,494		65,494		55,809
Corporate | In North America offices | Mortgage and real estate
Loans
Total loans, net of unearned income	62,162		62,162		60,675
Corporate | In North America offices | Installment and other
Loans
Total loans, net of unearned income	26,757		26,757		26,744
Corporate | In North America offices | Lease financing
Loans
Total loans, net of unearned income	547		547		673
Corporate | In offices outside North America
Loans
Total loans, net of unearned income	192,430		192,430		185,412
Corporate | In offices outside North America | Commercial and industrial
Loans
Total loans, net of unearned income	105,486		105,486		104,072
Corporate | In offices outside North America | Financial institutions
Loans
Total loans, net of unearned income	35,713		35,713		32,334
Corporate | In offices outside North America | Mortgage and real estate
Loans
Total loans, net of unearned income	10,995		10,995		11,371
Corporate | In offices outside North America | Installment and other
Loans
Total loans, net of unearned income	35,787		35,787		33,759
Corporate | In offices outside North America | Lease financing
Loans
Total loans, net of unearned income	54		54		65
Corporate | In offices outside North America | Government and official institutions
Loans
Total loans, net of unearned income	$ 4,395		$ 4,395		$ 3,811