LOANS - Consumer Loan Delinquency (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Non-Accrual Details
Loans, net of unearned income	$ 638,435	$ 617,617
Loans at fair value	3,970	5,005
Consumer
Non-Accrual Details
Loans, current	320,776	316,953
Loans, net of unearned income	328,702	325,785
Total non-accrual	3,552	3,658
90 days past due and accruing	$ 2,810	$ 3,537
Loans less than this number of days past due are considered current	30 days	30 days
Number of days past due, non-accrual status	60 days
Consumer | Less than or equal to 80%
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 60,450	$ 60,463
Consumer | 80% but less than or equal to 100%
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	7,273	9,625
Consumer | Greater than 100%
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	1,740	3,153
Consumer | Less than 620
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	12,554	13,228
Consumer | ≥ 620 but less than 660
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	14,870	15,160
Consumer | Equal to or greater than 660
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	167,233	158,740
Consumer | Government-guaranteed
Non-Accrual Details
Loans, past due	1,499	2,318
Consumer | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	3,401	3,461
Consumer | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	3,026	3,053
Consumer | Residential first mortgages
Non-Accrual Details
Loans at fair value	$ 31	$ 34
Number of days past due, non-accrual status	90 days	90 days
Consumer | Residential first mortgages | Less than or equal to 80%
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 47,092	$ 46,559
Consumer | Residential first mortgages | 80% but less than or equal to 100%
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	3,299	4,478
Consumer | Residential first mortgages | Greater than 100%
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	315	626
Consumer | Residential first mortgages | Less than 620
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	2,817	3,483
Consumer | Residential first mortgages | ≥ 620 but less than 660
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	2,615	3,036
Consumer | Residential first mortgages | Equal to or greater than 660
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	45,203	45,047
Consumer | Home equity loans | Less than or equal to 80%
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	13,358	13,904
Consumer | Home equity loans | 80% but less than or equal to 100%
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	3,974	5,147
Consumer | Home equity loans | Greater than 100%
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	1,425	2,527
Consumer | Home equity loans | Less than 620
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	1,751	2,067
Consumer | Home equity loans | ≥ 620 but less than 660
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	1,502	1,782
Consumer | Home equity loans | Equal to or greater than 660
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	15,600	17,837
Consumer | Credit cards | Less than 620
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	7,660	7,341
Consumer | Credit cards | ≥ 620 but less than 660
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	10,484	10,072
Consumer | Credit cards | Equal to or greater than 660
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	103,781	93,194
Consumer | Installment and other revolving | Less than 620
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	326	337
Consumer | Installment and other revolving | ≥ 620 but less than 660
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	269	270
Consumer | Installment and other revolving | Equal to or greater than 660
Financing Receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 2,649	2,662
Consumer | Commercial banking loans
Non-Accrual Details
Number of days past due, non-accrual status	90 days
Consumer | Total GCB and Citi Holdings consumer
Non-Accrual Details
Loans, current	$ 320,772	316,789
Loans, net of unearned income	328,698	325,607
Total non-accrual	3,551	3,633
90 days past due and accruing	2,810	3,537
Consumer | Total GCB and Citi Holdings consumer | Government-guaranteed
Non-Accrual Details
Loans, past due	1,499	2,318
Consumer | Total GCB and Citi Holdings consumer | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	3,401	3,454
Consumer | Total GCB and Citi Holdings consumer | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	3,026	3,046
Consumer | Other
Non-Accrual Details
Loans, current	4	164
Loans, net of unearned income	4	178
Total non-accrual	1	25
90 days past due and accruing	0	0
Consumer | Other | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | Other | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	0	7
Consumer | Other | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	0	7
Consumer | In North America offices
Non-Accrual Details
Loans, current	207,456	199,112
Loans, net of unearned income	213,547	205,982
Total non-accrual	2,429	2,511
90 days past due and accruing	2,550	3,259
Consumer | In North America offices | Government-guaranteed
Non-Accrual Details
Loans, past due	1,499	2,318
Consumer | In North America offices | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	2,372	2,374
Consumer | In North America offices | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	2,220	2,178
Consumer | In North America offices | Residential first mortgages
Non-Accrual Details
Loans, current	52,266	53,146
Loans, net of unearned income	54,665	56,874
Total non-accrual	1,225	1,216
90 days past due and accruing	1,267	1,997
Consumer | In North America offices | Residential first mortgages | Government-guaranteed
Non-Accrual Details
Loans, past due	1,499	2,318
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	564	846
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due | Government-guaranteed
Non-Accrual Details
Loans, past due	200	300
Consumer | In North America offices | Residential first mortgages | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	336	564
Consumer | In North America offices | Residential first mortgages | Equal to greater than 90 days past due | Government-guaranteed
Non-Accrual Details
Loans, past due	1,300	2,000
Consumer | In North America offices | Home equity loans
Non-Accrual Details
Loans, current	19,324	22,335
Loans, net of unearned income	20,019	22,748
Total non-accrual	728	1,017
90 days past due and accruing	0	0
Consumer | In North America offices | Home equity loans | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In North America offices | Home equity loans | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	261	136
Consumer | In North America offices | Home equity loans | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	434	277
Consumer | In North America offices | Credit cards
Non-Accrual Details
Loans, current	122,592	110,814
Loans, net of unearned income	125,323	113,353
Total non-accrual	0	0
90 days past due and accruing	1,271	1,243
Consumer | In North America offices | Credit cards | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In North America offices | Credit cards | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	1,460	1,296
Consumer | In North America offices | Credit cards | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	1,271	1,243
Consumer | In North America offices | Installment and other revolving
Non-Accrual Details
Loans, current	4,647	4,576
Loans, net of unearned income	4,749	4,689
Total non-accrual	69	56
90 days past due and accruing	0	2
Consumer | In North America offices | Installment and other revolving | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In North America offices | Installment and other revolving | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	64	80
Consumer | In North America offices | Installment and other revolving | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	38	33
Consumer | In North America offices | Commercial banking loans
Non-Accrual Details
Loans, current	8,627	8,241
Loans, net of unearned income	8,791	8,318
Total non-accrual	407	222
90 days past due and accruing	12	17
Consumer | In North America offices | Commercial banking loans | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In North America offices | Commercial banking loans | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	23	16
Consumer | In North America offices | Commercial banking loans | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	141	61
Consumer | In offices outside North America
Non-Accrual Details
Loans, current	113,316	117,677
Loans, net of unearned income	115,151	119,625
Total non-accrual	1,122	1,122
90 days past due and accruing	260	278
Consumer | In offices outside North America | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In offices outside North America | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	1,029	1,080
Consumer | In offices outside North America | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	806	868
Consumer | In offices outside North America | Residential first mortgages
Non-Accrual Details
Loans, current	38,433	39,551
Loans, net of unearned income	38,835	39,966
Total non-accrual	392	388
90 days past due and accruing	0	0
Consumer | In offices outside North America | Residential first mortgages | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In offices outside North America | Residential first mortgages | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	244	240
Consumer | In offices outside North America | Residential first mortgages | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	158	175
Consumer | In offices outside North America | Credit cards
Non-Accrual Details
Loans, current	24,270	25,698
Loans, net of unearned income	25,099	26,617
Total non-accrual	258	261
90 days past due and accruing	260	278
Consumer | In offices outside North America | Credit cards | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In offices outside North America | Credit cards | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	438	477
Consumer | In offices outside North America | Credit cards | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	391	442
Consumer | In offices outside North America | Installment and other revolving
Non-Accrual Details
Loans, current	25,632	27,664
Loans, net of unearned income	26,106	28,201
Total non-accrual	314	226
90 days past due and accruing	0	0
Consumer | In offices outside North America | Installment and other revolving | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In offices outside North America | Installment and other revolving | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	334	317
Consumer | In offices outside North America | Installment and other revolving | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	140	220
Consumer | In offices outside North America | Commercial banking loans
Non-Accrual Details
Loans, current	24,981	24,764
Loans, net of unearned income	25,111	24,841
Total non-accrual	158	247
90 days past due and accruing	0	0
Consumer | In offices outside North America | Commercial banking loans | Government-guaranteed
Non-Accrual Details
Loans, past due	0	0
Consumer | In offices outside North America | Commercial banking loans | 30 to 89 Days Past Due
Non-Accrual Details
Loans, past due	13	46
Consumer | In offices outside North America | Commercial banking loans | Equal to greater than 90 days past due
Non-Accrual Details
Loans, past due	$ 117	$ 31