LOANS - Consumer Loans (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) category	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) category	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Loans
Number of loan categories | category	2		2
Loans, net of unearned income	$ 674,909		$ 674,909		$ 667,034
Consumer
Loans
Total loans	324,757		324,757		332,919
Net unearned income	712		712		737
Loans, net of unearned income	325,469		325,469		333,656
Loans sold and/or reclassified to held-for-sale	300	$ 400	3,000	$ 3,200
In U.S. offices | Consumer
Loans
Total loans	209,300		209,300		215,711
In U.S. offices | Consumer | Mortgage and real estate
Loans
Total loans	61,048		61,048		65,467
In U.S. offices | Consumer | Installment, revolving credit and other
Loans
Total loans	3,515		3,515		3,398
In U.S. offices | Consumer | Cards
Loans
Total loans	137,051		137,051		139,006
In U.S. offices | Consumer | Commercial and industrial
Loans
Total loans	7,686		7,686		7,840
In offices outside the U.S. | Consumer
Loans
Total loans	115,457		115,457		117,208
In offices outside the U.S. | Consumer | Mortgage and real estate
Loans
Total loans	43,714		43,714		44,081
In offices outside the U.S. | Consumer | Installment, revolving credit and other
Loans
Total loans	27,899		27,899		26,556
In offices outside the U.S. | Consumer | Cards
Loans
Total loans	24,971		24,971		26,257
In offices outside the U.S. | Consumer | Commercial and industrial
Loans
Total loans	18,821		18,821		20,238
In offices outside the U.S. | Consumer | Lease financing
Loans
Total loans	$ 52		$ 52		$ 76