LOANS - Corporate Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Loans
Total loans, net of unearned income	$ 664,764		$ 664,764		$ 675,883
Lease financing
Loans
Total loans, net of unearned income	500		500		700
Corporate
Loans
Total loans, net of unearned income	400,514		400,514		387,044
Unearned income	(831)		(831)		(844)
Loans sold and/or reclassified to held-for-sale	1,000	$ 600	4,100	$ 1,700
Corporate | Commercial and industrial
Loans
Total loans, net of unearned income	155,985		155,985		156,340
Corporate | Financial institutions
Loans
Total loans, net of unearned income	95,820		95,820		87,689
Corporate | Mortgage and real estate
Loans
Total loans, net of unearned income	74,261		74,261		72,038
Corporate | Lease financing
Loans
Total loans, net of unearned income	472		472		738
Corporate | In North America offices
Loans
Total loans, net of unearned income	210,245		210,245		201,632
Corporate | In North America offices | Commercial and industrial
Loans
Total loans, net of unearned income	56,496		56,496		57,731
Corporate | In North America offices | Financial institutions
Loans
Total loans, net of unearned income	62,818		62,818		55,809
Corporate | In North America offices | Mortgage and real estate
Loans
Total loans, net of unearned income	63,584		63,584		60,675
Corporate | In North America offices | Installment and other
Loans
Total loans, net of unearned income	26,922		26,922		26,744
Corporate | In North America offices | Lease financing
Loans
Total loans, net of unearned income	425		425		673
Corporate | In offices outside North America
Loans
Total loans, net of unearned income	190,269		190,269		185,412
Corporate | In offices outside North America | Commercial and industrial
Loans
Total loans, net of unearned income	105,671		105,671		104,072
Corporate | In offices outside North America | Financial institutions
Loans
Total loans, net of unearned income	33,501		33,501		32,334
Corporate | In offices outside North America | Mortgage and real estate
Loans
Total loans, net of unearned income	10,685		10,685		11,371
Corporate | In offices outside North America | Installment and other
Loans
Total loans, net of unearned income	36,054		36,054		33,759
Corporate | In offices outside North America | Lease financing
Loans
Total loans, net of unearned income	47		47		65
Corporate | In offices outside North America | Government and official institutions
Loans
Total loans, net of unearned income	$ 4,311		$ 4,311		$ 3,811