LOANS - Consumer Loan Delinquency (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015 USD ($) payment re-aging	Dec. 31, 2014 USD ($)
Loans receivable
Loans, net of unearned income	$ 632,118	$ 644,635
Loans at fair value	6,538	5,901
Residential first mortgages | Less than or equal to 80%
Loans receivable
Loans	48,757	48,163
Residential first mortgages | 80% but less than or equal to 100%
Loans receivable
Loans	7,466	9,480
Residential first mortgages | Greater than 100%
Loans receivable
Loans	1,960	2,670
Residential first mortgages | Less than 620
Loans receivable
Loans	7,381	8,911
Residential first mortgages | ≥ 620 but less than 660
Loans receivable
Loans	4,778	5,463
Residential first mortgages | Equal to or greater than 660
Loans receivable
Loans	45,885	45,783
Home equity loans | Less than or equal to 80%
Loans receivable
Loans	14,487	14,638
Home equity loans | 80% but less than or equal to 100%
Loans receivable
Loans	6,279	7,267
Home equity loans | Greater than 100%
Loans receivable
Loans	3,903	4,641
Home equity loans | Less than 620
Loans receivable
Loans	2,891	3,257
Home equity loans | ≥ 620 but less than 660
Loans receivable
Loans	2,277	2,456
Home equity loans | Equal to or greater than 660
Loans receivable
Loans	19,614	20,957
Credit cards | Less than 620
Loans receivable
Loans	6,853	7,647
Credit cards | ≥ 620 but less than 660
Loans receivable
Loans	9,543	10,296
Credit cards | Equal to or greater than 660
Loans receivable
Loans	88,141	92,877
Installment and other | Less than 620
Loans receivable
Loans	330	4,015
Installment and other | ≥ 620 but less than 660
Loans receivable
Loans	253	2,520
Installment and other | Equal to or greater than 660
Loans receivable
Loans	$ 2,570	5,150
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	$ 332,442	357,348
Loans, net of unearned income	342,349	369,970
Loans, total non-accrual	5,279	5,924
Loans, 90 days past due and accruing	$ 3,539	$ 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	$ 63,244	$ 62,801
Consumer | 80% but less than or equal to 100%
Loans receivable
Loans	13,745	16,747
Consumer | Greater than 100%
Loans receivable
Loans	5,863	7,311
Consumer | Less than 620
Loans receivable
Loans	17,455	23,830
Consumer | ≥ 620 but less than 660
Loans receivable
Loans	16,851	20,735
Consumer | Equal to or greater than 660
Loans receivable
Loans	156,210	164,767
Consumer | Government-guaranteed
Loans receivable
Loans, past due	2,557	3,443
Consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,668	4,529
Consumer | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 3,682	$ 4,650
Consumer | Residential first mortgages
Loans receivable
Number of days past due, non-accrual status	90 days	90 days
Loans at fair value	$ 39	$ 43
Consumer | Home equity loans
Loans receivable
Number of days past due, non-accrual status	90 days
Consumer | Credit cards
Loans receivable
Number of days past due, non-accrual status	180 days
Consumer | Installment and other
Loans receivable
Number of days past due, non-accrual status	90 days
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	$ 332,229	357,110
Loans, net of unearned income	342,119	369,711
Loans, total non-accrual	5,248	5,894
Loans, 90 days past due and accruing	3,539	4,372
Consumer | Total GCB and Citi Holdings Consumer | Government-guaranteed
Loans receivable
Loans, past due	2,557	3,443
Consumer | Total GCB and Citi Holdings Consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,659	4,519
Consumer | Total GCB and Citi Holdings Consumer | Equal to greater than 90 days past due
Loans receivable
Loans, past due	3,674	4,639
Consumer | Other
Loans receivable
Loans, current	213	238
Loans, net of unearned income	230	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	9	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	$ 203,084	221,424
Loans, net of unearned income	210,758	231,517
Loans, total non-accrual	3,926	4,406
Loans, 90 days past due and accruing	3,236	4,050
Consumer | In North America Offices | Government-guaranteed
Loans receivable
Loans, past due	2,557	3,443
Consumer | In North America Offices | 30 to 89 Days Past Due
Loans receivable
Loans, past due	2,443	3,191
Consumer | In North America Offices | Equal to greater than 90 days past due
Loans receivable
Loans, past due	2,674	3,459
Consumer | In North America Offices | Residential first mortgages
Loans receivable
Loans, current	59,431	61,730
Loans, net of unearned income	64,006	67,824
Loans, total non-accrual	2,551	2,746
Loans, 90 days past due and accruing	2,161	2,759
Consumer | In North America Offices | Residential first mortgages | Government-guaranteed
Loans receivable
Loans, past due	2,557	3,443
Consumer | In North America Offices | Residential first mortgages | 30 to 89 Days Past Due
Loans receivable
Loans, past due	964	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	1,054	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	22,374	27,262
Loans, net of unearned income	23,137	28,117
Loans, total non-accrual	1,171	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	284	335
Consumer | In North America Offices | Home equity loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	479	520
Consumer | In North America Offices | Credit cards
Loans receivable
Loans, current	105,569	111,441
Loans, net of unearned income	107,745	114,028
Loans, total non-accrual	0	0
Loans, 90 days past due and accruing	1,062	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	1,114	1,316
Consumer | In North America Offices | Credit cards | Equal to greater than 90 days past due
Loans receivable
Loans, past due	1,062	1,271
Consumer | In North America Offices | Installment and other
Loans receivable
Loans, current	7,247	12,361
Loans, net of unearned income	7,347	12,874
Loans, total non-accrual	43	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	60	229
Consumer | In North America Offices | Installment and other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	40	284
Consumer | In North America Offices | Commercial market loans
Loans receivable
Loans, current	8,463	8,630
Loans, net of unearned income	8,523	8,674
Loans, total non-accrual	161	135
Loans, 90 days past due and accruing	9	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	21	31
Consumer | In North America Offices | Commercial market loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	39	13
Consumer | In offices outside North America
Loans receivable
Loans, current	129,145	135,686
Loans, net of unearned income	131,361	138,194
Loans, total non-accrual	1,322	1,488
Loans, 90 days past due and accruing	303	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	1,216	1,328
Consumer | In offices outside North America | Equal to greater than 90 days past due
Loans receivable
Loans, past due	1,000	1,180
Consumer | In offices outside North America | Residential first mortgages
Loans receivable
Loans, current	41,818	44,782
Loans, net of unearned income	42,272	45,317
Loans, total non-accrual	410	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	271	312
Consumer | In offices outside North America | Residential first mortgages | Equal to greater than 90 days past due
Loans receivable
Loans, past due	183	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	28,461	30,327
Loans, net of unearned income	29,502	31,482
Loans, total non-accrual	346	413
Loans, 90 days past due and accruing	303	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	544	602
Consumer | In offices outside North America | Credit cards | Equal to greater than 90 days past due
Loans receivable
Loans, past due	497	553
Consumer | In offices outside North America | Installment and other
Loans receivable
Loans, current	28,706	29,297
Loans, net of unearned income	29,159	29,774
Loans, total non-accrual	201	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	317	328
Consumer | In offices outside North America | Installment and other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	136	149
Consumer | In offices outside North America | Commercial market loans
Loans receivable
Loans, current	30,160	31,280
Loans, net of unearned income	30,428	31,621
Loans, total non-accrual	365	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	84	86
Consumer | In offices outside North America | Commercial market loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 184	$ 255