LOANS - Consumer Loan Delinquency (Details) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2016 USD ($) payment re-aging	Dec. 31, 2015 USD ($)
Loans receivable
Loans, net of unearned income	$ 618,824	$ 617,617
Loans at fair value	$ 4,793	5,005
Consumer
Loans receivable
Number of days past due, non-accrual status	60 days
Loans, current	$ 309,632	316,953
Loans, net of unearned income	317,900	325,785
Total non-accrual	3,601	3,658
90 days past due and accruing	$ 3,256	$ 3,537
Loans less than this number of days past due are considered current	30 days	30 days
Consumer | Minimum
Loans receivable
Minimum number of payments made consecutively for the loans to be re-aged | payment	1
Consumer | Maximum
Loans receivable
Minimum number of payments made consecutively for the loans to be re-aged | payment	3
Consumer | Less than or equal to 80%
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 59,427	$ 60,463
Consumer | 80% but less than or equal to 100%
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	9,847	9,625
Consumer | Greater than 100%
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	3,281	3,153
Consumer | Less than 620
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	13,186	13,228
Consumer | ≥ 620 but less than 660
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	14,832	15,160
Consumer | Equal to or greater than 660
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	152,346	158,740
Consumer | Government-guaranteed
Loans receivable
Loans, past due	2,049	2,318
Consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,295	3,461
Consumer | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 2,924	$ 3,053
Consumer | Residential first mortgages
Loans receivable
Number of days past due, non-accrual status	90 days	90 days
Loans at fair value	$ 33	$ 34
Consumer | Residential first mortgages | Less than or equal to 80%
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	46,181	46,559
Consumer | Residential first mortgages | 80% but less than or equal to 100%
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	5,034	4,478
Consumer | Residential first mortgages | Greater than 100%
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	720	626
Consumer | Residential first mortgages | Less than 620
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	3,387	3,483
Consumer | Residential first mortgages | ≥ 620 but less than 660
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	3,024	3,036
Consumer | Residential first mortgages | Equal to or greater than 660
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 45,437	45,047
Consumer | Home equity loans
Loans receivable
Number of days past due, non-accrual status	90 days
Consumer | Home equity loans | Less than or equal to 80%
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 13,246	13,904
Consumer | Home equity loans | 80% but less than or equal to 100%
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	4,813	5,147
Consumer | Home equity loans | Greater than 100%
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	2,561	2,527
Consumer | Home equity loans | Less than 620
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	2,032	2,067
Consumer | Home equity loans | ≥ 620 but less than 660
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	1,700	1,782
Consumer | Home equity loans | Equal to or greater than 660
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 16,995	17,837
Consumer | Credit cards
Loans receivable
Number of days past due, non-accrual status	180 days
Consumer | Credit cards | Less than 620
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 7,430	7,341
Consumer | Credit cards | ≥ 620 but less than 660
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	9,837	10,072
Consumer | Credit cards | Equal to or greater than 660
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 87,333	93,194
Consumer | Unsecured revolving loans
Loans receivable
Number of days past due, non-accrual status	180 days
Consumer | Installment and other revolving
Loans receivable
Number of days past due, non-accrual status	90 days
Consumer | Installment and other revolving | Less than 620
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 337	337
Consumer | Installment and other revolving | ≥ 620 but less than 660
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	271	270
Consumer | Installment and other revolving | Equal to or greater than 660
Loans receivable
Loans acquired with deteriorated credit quality in accordance with ASC 310-30	$ 2,581	2,662
Consumer | Commercial banking loans
Loans receivable
Number of days past due, non-accrual status	90 days
Consumer | Total GCB and Citi Holdings consumer
Loans receivable
Loans, current	$ 309,476	316,789
Loans, net of unearned income	317,732	325,607
Total non-accrual	3,578	3,633
90 days past due and accruing	3,256	3,537
Consumer | Total GCB and Citi Holdings consumer | Government-guaranteed
Loans receivable
Loans, past due	2,049	2,318
Consumer | Total GCB and Citi Holdings consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,289	3,454
Consumer | Total GCB and Citi Holdings consumer | Equal to greater than 90 days past due
Loans receivable
Loans, past due	2,918	3,046
Consumer | Other
Loans receivable
Loans, current	156	164
Loans, net of unearned income	168	178
Total non-accrual	23	25
90 days past due and accruing	0	0
Consumer | Other | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | Other | 30 to 89 Days Past Due
Loans receivable
Loans, past due	6	7
Consumer | Other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 6	7
Consumer | Open-ended consumer loans
Loans receivable
Minimum number of payments made consecutively for the loans to be re-aged | payment	3
Number of re-age modification limitations in twelve months | re-aging	1
Number of re-age modification limitations in five years | re-aging	2
Consumer | In North America offices
Loans receivable
Loans, current	$ 193,255	199,112
Loans, net of unearned income	199,548	205,982
Total non-accrual	2,516	2,511
90 days past due and accruing	2,990	3,259
Consumer | In North America offices | Government-guaranteed
Loans receivable
Loans, past due	2,049	2,318
Consumer | In North America offices | 30 to 89 Days Past Due
Loans receivable
Loans, past due	2,146	2,374
Consumer | In North America offices | Equal to greater than 90 days past due
Loans receivable
Loans, past due	2,098	2,178
Consumer | In North America offices | Residential first mortgages
Loans receivable
Loans, current	53,482	53,146
Loans, net of unearned income	56,809	56,874
Total non-accrual	1,185	1,216
90 days past due and accruing	1,779	1,997
Consumer | In North America offices | Residential first mortgages | Government-guaranteed
Loans receivable
Loans, past due	2,049	2,318
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due
Loans receivable
Loans, past due	733	846
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due | Government-guaranteed
Loans receivable
Loans, past due	200	300
Consumer | In North America offices | Residential first mortgages | Equal to greater than 90 days past due
Loans receivable
Loans, past due	545	564
Consumer | In North America offices | Residential first mortgages | Equal to greater than 90 days past due | Government-guaranteed
Loans receivable
Loans, past due	1,800	2,000
Consumer | In North America offices | Home equity loans
Loans receivable
Loans, current	21,454	22,335
Loans, net of unearned income	21,899	22,748
Total non-accrual	990	1,017
90 days past due and accruing	0	0
Consumer | In North America offices | Home equity loans | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In North America offices | Home equity loans | 30 to 89 Days Past Due
Loans receivable
Loans, past due	164	136
Consumer | In North America offices | Home equity loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	281	277
Consumer | In North America offices | Credit cards
Loans receivable
Loans, current	105,118	110,814
Loans, net of unearned income	107,493	113,353
Total non-accrual	0	0
90 days past due and accruing	1,195	1,243
Consumer | In North America offices | Credit cards | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In North America offices | Credit cards | 30 to 89 Days Past Due
Loans receivable
Loans, past due	1,180	1,296
Consumer | In North America offices | Credit cards | Equal to greater than 90 days past due
Loans receivable
Loans, past due	1,195	1,243
Consumer | In North America offices | Installment and other revolving
Loans receivable
Loans, current	4,638	4,576
Loans, net of unearned income	4,726	4,689
Total non-accrual	60	56
90 days past due and accruing	0	2
Consumer | In North America offices | Installment and other revolving | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In North America offices | Installment and other revolving | 30 to 89 Days Past Due
Loans receivable
Loans, past due	56	80
Consumer | In North America offices | Installment and other revolving | Equal to greater than 90 days past due
Loans receivable
Loans, past due	32	33
Consumer | In North America offices | Commercial banking loans
Loans receivable
Loans, current	8,563	8,241
Loans, net of unearned income	8,621	8,318
Total non-accrual	281	222
90 days past due and accruing	16	17
Consumer | In North America offices | Commercial banking loans | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In North America offices | Commercial banking loans | 30 to 89 Days Past Due
Loans receivable
Loans, past due	13	16
Consumer | In North America offices | Commercial banking loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	45	61
Consumer | In offices outside North America
Loans receivable
Loans, current	116,221	117,677
Loans, net of unearned income	118,184	119,625
Total non-accrual	1,062	1,122
90 days past due and accruing	266	278
Consumer | In offices outside North America | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In offices outside North America | 30 to 89 Days Past Due
Loans receivable
Loans, past due	1,143	1,080
Consumer | In offices outside North America | Equal to greater than 90 days past due
Loans receivable
Loans, past due	820	868
Consumer | In offices outside North America | Residential first mortgages
Loans receivable
Loans, current	40,293	39,551
Loans, net of unearned income	40,767	39,966
Total non-accrual	389	388
90 days past due and accruing	0	0
Consumer | In offices outside North America | Residential first mortgages | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In offices outside North America | Residential first mortgages | 30 to 89 Days Past Due
Loans receivable
Loans, past due	310	240
Consumer | In offices outside North America | Residential first mortgages | Equal to greater than 90 days past due
Loans receivable
Loans, past due	164	175
Consumer | In offices outside North America | Credit cards
Loans receivable
Loans, current	24,774	25,698
Loans, net of unearned income	25,648	26,617
Total non-accrual	226	261
90 days past due and accruing	266	278
Consumer | In offices outside North America | Credit cards | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In offices outside North America | Credit cards | 30 to 89 Days Past Due
Loans receivable
Loans, past due	470	477
Consumer | In offices outside North America | Credit cards | Equal to greater than 90 days past due
Loans receivable
Loans, past due	404	442
Consumer | In offices outside North America | Installment and other revolving
Loans receivable
Loans, current	27,739	27,664
Loans, net of unearned income	28,287	28,201
Total non-accrual	213	226
90 days past due and accruing	0	0
Consumer | In offices outside North America | Installment and other revolving | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In offices outside North America | Installment and other revolving | 30 to 89 Days Past Due
Loans receivable
Loans, past due	325	317
Consumer | In offices outside North America | Installment and other revolving | Equal to greater than 90 days past due
Loans receivable
Loans, past due	223	220
Consumer | In offices outside North America | Commercial banking loans
Loans receivable
Loans, current	23,415	24,764
Loans, net of unearned income	23,482	24,841
Total non-accrual	234	247
90 days past due and accruing	0	0
Consumer | In offices outside North America | Commercial banking loans | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In offices outside North America | Commercial banking loans | 30 to 89 Days Past Due
Loans receivable
Loans, past due	38	46
Consumer | In offices outside North America | Commercial banking loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 29	$ 31