LOANS - Consumer Troubled Debt Restructurings (Details) - Consumer $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018 USD ($) loan	Sep. 30, 2017 USD ($) loan	Sep. 30, 2018 USD ($) loan	Sep. 30, 2017 USD ($) 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	62,563	61,847	183,804	177,919
Post-modification recorded investment	$ 350	$ 555	$ 1,066	$ 1,402
TDR in payment default	$ 95	$ 120	$ 323	$ 348
In North America offices | Residential first mortgages
Loans receivable
Number of loans modified | loan	461	1,400	1,544	3,172
Post-modification recorded investment	$ 66	$ 199	$ 233	$ 445
Average interest rate reduction	0.00%	0.00%	0.00%	1.00%
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 10	$ 12	$ 29	$ 42
TDR in payment default	31	57	105	156
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	$ 7	$ 7	$ 20	$ 28
In North America offices | Home equity loans
Loans receivable
Number of loans modified | loan	261	830	1,097	2,186
Post-modification recorded investment	$ 26	$ 70	$ 104	$ 185
Average interest rate reduction	1.00%	1.00%	1.00%	1.00%
Post-modification recorded investment for borrowers that have gone through Chapter 7 bankruptcy	$ 2	$ 5	$ 10	$ 16
TDR in payment default	5	8	21	25
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	$ 2	$ 5	$ 9	$ 14
In North America offices | Credit cards
Loans receivable
Number of loans modified | loan	61,508	59,285	180,170	171,702
Post-modification recorded investment	$ 253	$ 225	$ 717	$ 659
Average interest rate reduction	18.00%	17.00%	17.00%	17.00%
TDR in payment default	$ 57	$ 54	$ 173	$ 163
In North America offices | Installment and other revolving
Loans receivable
Number of loans modified | loan	322	299	956	770
Post-modification recorded investment	$ 2	$ 2	$ 7	$ 6
Average interest rate reduction	5.00%	6.00%	5.00%	5.00%
TDR in payment default	$ 1	$ 1	$ 2	$ 2
In North America offices | Commercial banking
Loans receivable
Number of loans modified | loan	11	33	37	89
Post-modification recorded investment	$ 3	$ 59	$ 5	$ 107
Average interest rate reduction	0.00%	0.00%	0.00%	0.00%
TDR in payment default	$ 1	$ 0	$ 22	$ 2
In offices outside the U.S.
Loans receivable
Number of loans modified | loan	25,625	40,779	84,773	118,949
Post-modification recorded investment	$ 142	$ 209	$ 507	$ 590
TDR in payment default	$ 75	$ 76	$ 241	$ 215
In offices outside the U.S. | Residential first mortgages
Loans receivable
Number of loans modified | loan	660	703	1,833	2,071
Post-modification recorded investment	$ 22	$ 25	$ 62	$ 80
Average interest rate reduction	0.00%	0.00%	0.00%	0.00%
TDR in payment default	$ 2	$ 3	$ 6	$ 8
In offices outside the U.S. | Credit cards
Loans receivable
Number of loans modified | loan	18,413	28,254	59,589	82,042
Post-modification recorded investment	$ 77	$ 103	$ 249	$ 286
Average interest rate reduction	17.00%	11.00%	16.00%	12.00%
TDR in payment default	$ 48	$ 48	$ 156	$ 136
In offices outside the U.S. | Installment and other revolving
Loans receivable
Number of loans modified | loan	6,421	11,725	22,918	34,654
Post-modification recorded investment	$ 34	$ 70	$ 136	$ 194
Average interest rate reduction	10.00%	11.00%	10.00%	9.00%
TDR in payment default	$ 18	$ 25	$ 62	$ 71
In offices outside the U.S. | Commercial banking
Loans receivable
Number of loans modified | loan	131	97	433	182
Post-modification recorded investment	$ 9	$ 11	$ 60	$ 30
Average interest rate reduction	0.00%	0.00%	1.00%	0.00%
TDR in payment default	$ 7	$ 0	$ 17	$ 0
Deferred principal | In North America offices
Loans receivable
Post-modification recorded investment	1	6	6	18
Deferred principal | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	1	2	5
Deferred principal | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	1	5	4	13
Deferred principal | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0	0	0	0
Deferred principal | In North America offices | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0	0	0
Deferred principal | In North America offices | Commercial banking
Loans receivable
Post-modification recorded investment	0	0	0	0
Deferred principal | In offices outside the U.S.
Loans receivable
Post-modification recorded investment	0	0	0	0
Deferred principal | In offices outside the U.S. | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0	0	0
Deferred principal | In offices outside the U.S. | Credit cards
Loans receivable
Post-modification recorded investment	0	0	0	0
Deferred principal | In offices outside the U.S. | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0	0	0
Deferred principal | In offices outside the U.S. | Commercial banking
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In North America offices
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In North America offices | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In North America offices | Commercial banking
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In offices outside the U.S.
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In offices outside the U.S. | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In offices outside the U.S. | Credit cards
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In offices outside the U.S. | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0	0	0
Contingent principal forgiveness | In offices outside the U.S. | Commercial banking
Loans receivable
Post-modification recorded investment	0	0	0	0
Principal forgiveness | In North America offices
Loans receivable
Post-modification recorded investment	0	0	0	2
Principal forgiveness | In North America offices | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0	0	2
Principal forgiveness | In North America offices | Home equity loans
Loans receivable
Post-modification recorded investment	0	0	0	0
Principal forgiveness | In North America offices | Credit cards
Loans receivable
Post-modification recorded investment	0	0	0	0
Principal forgiveness | In North America offices | Installment and other revolving
Loans receivable
Post-modification recorded investment	0	0	0	0
Principal forgiveness | In North America offices | Commercial banking
Loans receivable
Post-modification recorded investment	0	0	0	0
Principal forgiveness | In offices outside the U.S.
Loans receivable
Post-modification recorded investment	4	5	13	15
Principal forgiveness | In offices outside the U.S. | Residential first mortgages
Loans receivable
Post-modification recorded investment	0	0	0	0
Principal forgiveness | In offices outside the U.S. | Credit cards
Loans receivable
Post-modification recorded investment	2	2	7	6
Principal forgiveness | In offices outside the U.S. | Installment and other revolving
Loans receivable
Post-modification recorded investment	2	3	6	9
Principal forgiveness | In offices outside the U.S. | Commercial banking
Loans receivable
Post-modification recorded investment	$ 0	$ 0	$ 0	$ 0