LOANS - Consumer Troubled Debt Restructurings (Details) - Consumer $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) loan	Mar. 31, 2017 USD ($) loan	Dec. 31, 2017
Loans receivable
Period within which default occurred post-modification	1 year
Number of days past due, non-accrual status	60 days
Residential first mortgages
Loans receivable
Number of days past due, non-accrual status	90 days		90 days
Commercial banking
Loans receivable
Number of days past due, non-accrual status	90 days
In North America offices
Loans receivable
Number of loans modified | loan	64,598	61,229
Post-modification recorded investment	$ 378	$ 424
TDR in payment default	$ 122	$ 114
In North America offices | Residential first mortgages
Loans receivable
Number of loans modified | loan	588	966
Post-modification recorded investment	$ 89	$ 130
Average interest rate reduction	0.00%	1.00%
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 11	$ 15
TDR in payment default	44	51
In North America offices | Residential first mortgages | New OCC guidance
Loans receivable
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 8	$ 9
In North America offices | Home equity loans
Loans receivable
Number of loans modified | loan	456	679
Post-modification recorded investment	$ 41	$ 56
Average interest rate reduction	1.00%	1.00%
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 4	$ 6
TDR in payment default	10	9
In North America offices | Home equity loans | New OCC guidance
Loans receivable
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 3	$ 6
In North America offices | Credit cards
Loans receivable
Number of loans modified | loan	63,203	59,337
Post-modification recorded investment	$ 244	$ 231
Average interest rate reduction	18.00%	17.00%
TDR in payment default	$ 59	$ 52
In North America offices | Installment and other revolving
Loans receivable
Number of loans modified | loan	342	221
Post-modification recorded investment	$ 3	$ 2
Average interest rate reduction	5.00%	5.00%
TDR in payment default	$ 1	$ 0
In North America offices | Commercial banking
Loans receivable
Number of loans modified | loan	9	26
Post-modification recorded investment	$ 1	$ 5
Average interest rate reduction	0.00%	0.00%
TDR in payment default	$ 8	$ 2
In offices outside the U.S.
Loans receivable
Number of loans modified | loan	33,413	37,189
Post-modification recorded investment	$ 199	$ 185
TDR in payment default	$ 79	$ 67
In offices outside the U.S. | Residential first mortgages
Loans receivable
Number of loans modified | loan	549	613
Post-modification recorded investment	$ 18	$ 27
Average interest rate reduction	0.00%	0.00%
TDR in payment default	$ 2	$ 2
In offices outside the U.S. | Credit cards
Loans receivable
Number of loans modified | loan	23,394	25,237
Post-modification recorded investment	$ 94	$ 85
Average interest rate reduction	15.00%	14.00%
TDR in payment default	$ 53	$ 42
In offices outside the U.S. | Installment and other revolving
Loans receivable
Number of loans modified | loan	9,325	11,307
Post-modification recorded investment	$ 59	$ 60
Average interest rate reduction	10.00%	7.00%
TDR in payment default	$ 24	$ 23
In offices outside the U.S. | Commercial banking
Loans receivable
Number of loans modified | loan	145	32
Post-modification recorded investment	$ 28	$ 13
Average interest rate reduction	2000000.00%	2.00%
TDR in payment default	$ 0	$ 0
Deferred principal | In North America offices
Loans receivable
Post-modification recorded investment	3	6
Deferred principal | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	1	3
Deferred principal | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	2	3
Deferred principal | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In North America offices | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In North America offices | Commercial banking
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In offices outside the U.S.
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In offices outside the U.S. | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In offices outside the U.S. | Credit cards
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In offices outside the U.S. | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0
Deferred principal | In offices outside the U.S. | Commercial banking
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In North America offices | Commercial banking
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In offices outside the U.S.
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In offices outside the U.S. | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In offices outside the U.S. | Credit cards
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In offices outside the U.S. | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0
Contingent principal forgiveness | In offices outside the U.S. | Commercial banking
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In North America offices
Loans receivable
Post-modification recorded investment	0	1
Principal forgiveness | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	1
Principal forgiveness | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In North America offices | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In North America offices | Commercial banking
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In offices outside the U.S.
Loans receivable
Post-modification recorded investment	4	6
Principal forgiveness | In offices outside the U.S. | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0
Principal forgiveness | In offices outside the U.S. | Credit cards
Loans receivable
Post-modification recorded investment	2	2
Principal forgiveness | In offices outside the U.S. | Installment and other revolving
Loans receivable
Post-modification recorded investment	2	4
Principal forgiveness | In offices outside the U.S. | Commercial banking
Loans receivable
Post-modification recorded investment	$ 0	$ 0