LOANS - Consumer Loans (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018 USD ($) category	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) category	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Loans
Number of loan categories | category	2		2
Loans, net of unearned income	$ 671,180		$ 671,180		$ 667,034
Consumer
Loans
Total loans	322,921		322,921		332,919
Net unearned income	711		711		737
Loans, net of unearned income	323,632		323,632		333,656
Loans sold and/or reclassified to held-for-sale	1,900	$ 600	2,800	$ 2,800
In U.S. offices | Consumer
Loans
Total loans	208,878		208,878		215,711
In U.S. offices | Consumer | Mortgage and real estate
Loans
Total loans	61,692		61,692		65,467
In U.S. offices | Consumer | Installment, revolving credit and other
Loans
Total loans	3,759		3,759		3,398
In U.S. offices | Consumer | Cards
Loans
Total loans	135,968		135,968		139,006
In U.S. offices | Consumer | Commercial and industrial
Loans
Total loans	7,459		7,459		7,840
In offices outside the U.S. | Consumer
Loans
Total loans	114,043		114,043		117,208
In offices outside the U.S. | Consumer | Mortgage and real estate
Loans
Total loans	43,056		43,056		44,081
In offices outside the U.S. | Consumer | Installment, revolving credit and other
Loans
Total loans	27,254		27,254		26,556
In offices outside the U.S. | Consumer | Cards
Loans
Total loans	24,712		24,712		26,257
In offices outside the U.S. | Consumer | Commercial and industrial
Loans
Total loans	18,966		18,966		20,238
In offices outside the U.S. | Consumer | Lease financing
Loans
Total loans	$ 55		$ 55		$ 76