LOANS - Consumer Loan Delinquency (Details) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) payment re-aging	Dec. 31, 2014 USD ($)
Loans receivable
Loans, net of unearned income	$ 622,444	$ 644,635
Loans at fair value	$ 5,513	5,901
Open-ended consumer loans
Loans receivable
Minimum number of payments made consecutively for the loans to be re-aged | payment	3
Consumer
Loans receivable
Number of days past due, non-accrual status	60 days
Loans, current	$ 323,404	357,348
Loans, net of unearned income	333,373	369,970
Loans, total non-accrual	4,849	5,924
Loans, 90 days past due and accruing	$ 3,584	$ 4,372
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	$ 65,330	$ 62,801
Consumer | 80% but less than or equal to 100%
Loans receivable
Loans	11,980	16,747
Consumer | Greater than 100%
Loans receivable
Loans	4,198	7,311
Consumer | Less than 620
Loans receivable
Loans	16,848	23,830
Consumer | ≥ 620 but less than 660
Loans receivable
Loans	16,655	20,735
Consumer | Equal to or greater than 660
Loans receivable
Loans	156,165	164,767
Consumer | Government-guaranteed
Loans receivable
Loans, past due	2,582	3,443
Consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,863	4,529
Consumer | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 3,524	$ 4,650
Consumer | Residential first mortgages
Loans receivable
Number of days past due, non-accrual status	90 days	90 days
Loans at fair value	$ 37	$ 43
Consumer | Residential first mortgages | Less than or equal to 80%
Loans receivable
Loans	50,484	48,163
Consumer | Residential first mortgages | 80% but less than or equal to 100%
Loans receivable
Loans	6,001	9,480
Consumer | Residential first mortgages | Greater than 100%
Loans receivable
Loans	1,285	2,670
Consumer | Residential first mortgages | Less than 620
Loans receivable
Loans	6,686	8,911
Consumer | Residential first mortgages | ≥ 620 but less than 660
Loans receivable
Loans	4,472	5,463
Consumer | Residential first mortgages | Equal to or greater than 660
Loans receivable
Loans	$ 46,462	45,783
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	$ 14,846	14,638
Consumer | Home equity loans | 80% but less than or equal to 100%
Loans receivable
Loans	5,979	7,267
Consumer | Home equity loans | Greater than 100%
Loans receivable
Loans	2,913	4,641
Consumer | Home equity loans | Less than 620
Loans receivable
Loans	2,730	3,257
Consumer | Home equity loans | ≥ 620 but less than 660
Loans receivable
Loans	2,196	2,456
Consumer | Home equity loans | Equal to or greater than 660
Loans receivable
Loans	$ 18,924	20,957
Consumer | Credit cards
Loans receivable
Number of days past due, non-accrual status	180 days
Consumer | Credit cards | Less than 620
Loans receivable
Loans	$ 7,087	7,647
Consumer | Credit cards | ≥ 620 but less than 660
Loans receivable
Loans	9,709	10,296
Consumer | Credit cards | Equal to or greater than 660
Loans receivable
Loans	$ 88,159	92,877
Consumer | Installment and other
Loans receivable
Number of days past due, non-accrual status	90 days
Consumer | Installment and other | Less than 620
Loans receivable
Loans	$ 345	4,015
Consumer | Installment and other | ≥ 620 but less than 660
Loans receivable
Loans	278	2,520
Consumer | Installment and other | Equal to or greater than 660
Loans receivable
Loans	$ 2,620	5,150
Consumer | Commercial market loans
Loans receivable
Number of days past due, non-accrual status	90 days
Consumer | Total GCB and Citi Holdings Consumer
Loans receivable
Loans, current	$ 323,206	357,110
Loans, net of unearned income	333,159	369,711
Loans, total non-accrual	4,818	5,894
Loans, 90 days past due and accruing	3,584	4,372
Consumer | Total GCB and Citi Holdings Consumer | Government-guaranteed
Loans receivable
Loans, past due	2,582	3,443
Consumer | Total GCB and Citi Holdings Consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,855	4,519
Consumer | Total GCB and Citi Holdings Consumer | Equal to greater than 90 days past due
Loans receivable
Loans, past due	3,516	4,639
Consumer | Other
Loans receivable
Loans, current	198	238
Loans, net of unearned income	214	259
Loans, total non-accrual	31	30
Loans, 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	8	10
Consumer | Other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 8	11
Consumer | Open-ended consumer loans
Loans receivable
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 | Unsecured Revolving Loans
Loans receivable
Number of days past due, non-accrual status	180 days
Consumer | In North America Offices
Loans receivable
Loans, current	$ 201,301	221,424
Loans, net of unearned income	209,177	231,517
Loans, total non-accrual	3,620	4,406
Loans, 90 days past due and accruing	3,309	4,050
Consumer | In North America Offices | Government-guaranteed
Loans receivable
Loans, past due	2,582	3,443
Consumer | In North America Offices | 30 to 89 Days Past Due
Loans receivable
Loans, past due	2,690	3,191
Consumer | In North America Offices | Equal to greater than 90 days past due
Loans receivable
Loans, past due	2,604	3,459
Consumer | In North America Offices | Residential first mortgages
Loans receivable
Loans, current	59,012	61,730
Loans, net of unearned income	63,542	67,824
Loans, total non-accrual	2,307	2,746
Loans, 90 days past due and accruing	2,180	2,759
Consumer | In North America Offices | Residential first mortgages | Government-guaranteed
Loans receivable
Loans, past due	2,582	3,443
Consumer | In North America Offices | Residential first mortgages | 30 to 89 Days Past Due
Loans receivable
Loans, past due	998	1,280
Consumer | In North America Offices | Residential first mortgages | 30 to 89 Days Past Due | Government-guaranteed
Loans receivable
Loans, past due	400	600
Consumer | In North America Offices | Residential first mortgages | Equal to greater than 90 days past due
Loans receivable
Loans, past due	950	1,371
Consumer | In North America Offices | Residential first mortgages | Equal to greater than 90 days past due | Government-guaranteed
Loans receivable
Loans, past due	2,200	2,800
Consumer | In North America Offices | Home equity loans
Loans receivable
Loans, current	24,258	27,262
Loans, net of unearned income	25,043	28,117
Loans, total non-accrual	1,125	1,271
Loans, 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	322	335
Consumer | In North America Offices | Home equity loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	463	520
Consumer | In North America Offices | Credit cards
Loans receivable
Loans, current	105,489	111,441
Loans, net of unearned income	107,863	114,028
Loans, total non-accrual	0	0
Loans, 90 days past due and accruing	1,112	1,273
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	1,262	1,316
Consumer | In North America Offices | Credit cards | Equal to greater than 90 days past due
Loans receivable
Loans, past due	1,112	1,271
Consumer | In North America Offices | Installment and other
Loans receivable
Loans, current	4,248	12,361
Loans, net of unearned income	4,359	12,874
Loans, total non-accrual	0	254
Loans, 90 days past due and accruing	4	3
Consumer | In North America Offices | Installment and other | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In North America Offices | Installment and other | 30 to 89 Days Past Due
Loans receivable
Loans, past due	74	229
Consumer | In North America Offices | Installment and other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	37	284
Consumer | In North America Offices | Commercial market loans
Loans receivable
Loans, current	8,294	8,630
Loans, net of unearned income	8,370	8,674
Loans, total non-accrual	188	135
Loans, 90 days past due and accruing	13	15
Consumer | In North America Offices | Commercial market loans | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In North America Offices | Commercial market loans | 30 to 89 Days Past Due
Loans receivable
Loans, past due	34	31
Consumer | In North America Offices | Commercial market loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	42	13
Consumer | In offices outside North America
Loans receivable
Loans, current	121,905	135,686
Loans, net of unearned income	123,982	138,194
Loans, total non-accrual	1,198	1,488
Loans, 90 days past due and accruing	275	322
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	1,165	1,328
Consumer | In offices outside North America | Equal to greater than 90 days past due
Loans receivable
Loans, past due	912	1,180
Consumer | In offices outside North America | Residential first mortgages
Loans receivable
Loans, current	40,296	44,782
Loans, net of unearned income	40,675	45,317
Loans, total non-accrual	381	454
Loans, 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	291	312
Consumer | In offices outside North America | Residential first mortgages | Equal to greater than 90 days past due
Loans receivable
Loans, past due	88	223
Consumer | In offices outside North America | Home equity loans
Loans receivable
Loans, current	0	0
Loans, net of unearned income	0	0
Loans, total non-accrual	0	0
Loans, 90 days past due and accruing	0	0
Consumer | In offices outside North America | Home equity loans | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In offices outside North America | Home equity loans | 30 to 89 Days Past Due
Loans receivable
Loans, past due	0	0
Consumer | In offices outside North America | Home equity loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	0	0
Consumer | In offices outside North America | Credit cards
Loans receivable
Loans, current	25,286	30,327
Loans, net of unearned income	26,216	31,482
Loans, total non-accrual	267	413
Loans, 90 days past due and accruing	275	322
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	499	602
Consumer | In offices outside North America | Credit cards | Equal to greater than 90 days past due
Loans receivable
Loans, past due	431	553
Consumer | In offices outside North America | Installment and other
Loans receivable
Loans, current	28,513	29,297
Loans, net of unearned income	29,141	29,774
Loans, total non-accrual	229	216
Loans, 90 days past due and accruing	0	0
Consumer | In offices outside North America | Installment and other | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In offices outside North America | Installment and other | 30 to 89 Days Past Due
Loans receivable
Loans, past due	321	328
Consumer | In offices outside North America | Installment and other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	307	149
Consumer | In offices outside North America | Commercial market loans
Loans receivable
Loans, current	27,810	31,280
Loans, net of unearned income	27,950	31,621
Loans, total non-accrual	321	405
Loans, 90 days past due and accruing	0	0
Consumer | In offices outside North America | Commercial market loans | Government-guaranteed
Loans receivable
Loans, past due	0	0
Consumer | In offices outside North America | Commercial market loans | 30 to 89 Days Past Due
Loans receivable
Loans, past due	54	86
Consumer | In offices outside North America | Commercial market loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 86	$ 255