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LOANS - Corporate Loans (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
LOANS - Corporate Loans (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Loans
Loans, net of unearned income	$ 666,911		$ 666,911		$ 699,483
Lease financing
Loans
Loans, net of unearned income	800		800		1,400
Corporate
Loans
Loans, net of unearned income	386,886		386,886		389,935
Unearned income	(857)		(857)		(814)
Loans sold and/or reclassified to held-for-sale	600	$ 800	1,700	$ 2,100
Corporate \| Commercial and industrial
Loans
Loans, net of unearned income	165,035		165,035		166,846
Corporate \| Financial institutions
Loans
Loans, net of unearned income	84,913		84,913		91,852
Corporate \| Mortgage and real estate
Loans
Loans, net of unearned income	69,761		69,761		63,151
Corporate \| Lease financing
Loans
Loans, net of unearned income	767		767		1,385
Corporate \| In North America offices
Loans
Loans, net of unearned income	199,707		199,707		195,750
Corporate \| In North America offices \| Commercial and industrial
Loans
Loans, net of unearned income	59,921		59,921		55,929
Corporate \| In North America offices \| Financial institutions
Loans
Loans, net of unearned income	52,884		52,884		53,922
Corporate \| In North America offices \| Mortgage and real estate
Loans
Loans, net of unearned income	59,340		59,340		53,371
Corporate \| In North America offices \| Installment and other
Loans
Loans, net of unearned income	26,858		26,858		31,238
Corporate \| In North America offices \| Lease financing
Loans
Loans, net of unearned income	704		704		1,290
Corporate \| In offices outside North America
Loans
Loans, net of unearned income	187,179		187,179		194,185
Corporate \| In offices outside North America \| Commercial and industrial
Loans
Loans, net of unearned income	108,551		108,551		112,668
Corporate \| In offices outside North America \| Financial institutions
Loans
Loans, net of unearned income	32,583		32,583		40,211
Corporate \| In offices outside North America \| Mortgage and real estate
Loans
Loans, net of unearned income	10,424		10,424		9,780
Corporate \| In offices outside North America \| Installment and other
Loans
Loans, net of unearned income	32,323		32,323		27,303
Corporate \| In offices outside North America \| Lease financing
Loans
Loans, net of unearned income	63		63		95
Corporate \| In offices outside North America \| Government and official institutions
Loans
Loans, net of unearned income	$ 3,235		$ 3,235		$ 4,128