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LOANS - Corporate Loans (Details) - USD ($) $ in Millions	3 Months Ended
LOANS - Corporate Loans (Details) - USD ($) $ in Millions	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Loans
Loans, net of unearned income	$ 721,020		$ 699,483
Lease financing
Loans
Loans, net of unearned income	1,100		1,400
Corporate
Loans
Loans, net of unearned income	432,590		389,935
Unearned income	791		814
Loans sold and/or reclassified to held-for-sale	200	$ 500
Corporate \| Commercial and industrial
Loans
Loans, net of unearned income	200,297		166,846
Corporate \| Financial institutions
Loans
Loans, net of unearned income	96,006		91,852
Corporate \| Mortgage and real estate
Loans
Loans, net of unearned income	65,217		63,151
Corporate \| Lease financing
Loans
Loans, net of unearned income	1,060		1,385
Corporate \| In North America offices
Loans
Loans, net of unearned income	228,891		195,750
Corporate \| In North America offices \| Commercial and industrial
Loans
Loans, net of unearned income	81,231		55,929
Corporate \| In North America offices \| Financial institutions
Loans
Loans, net of unearned income	60,653		53,922
Corporate \| In North America offices \| Mortgage and real estate
Loans
Loans, net of unearned income	55,428		53,371
Corporate \| In North America offices \| Installment and other
Loans
Loans, net of unearned income	30,591		31,238
Corporate \| In North America offices \| Lease financing
Loans
Loans, net of unearned income	988		1,290
Corporate \| In offices outside North America
Loans
Loans, net of unearned income	203,699		194,185
Corporate \| In offices outside North America \| Commercial and industrial
Loans
Loans, net of unearned income	121,703		112,668
Corporate \| In offices outside North America \| Financial institutions
Loans
Loans, net of unearned income	37,003		40,211
Corporate \| In offices outside North America \| Mortgage and real estate
Loans
Loans, net of unearned income	9,639		9,780
Corporate \| In offices outside North America \| Installment and other
Loans
Loans, net of unearned income	31,728		27,303
Corporate \| In offices outside North America \| Lease financing
Loans
Loans, net of unearned income	72		95
Corporate \| In offices outside North America \| Government and official institutions
Loans
Loans, net of unearned income	$ 3,554		$ 4,128