LOANS - Consumer Loan Delinquency (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) payment re-aging	Dec. 31, 2016 USD ($)
Loans receivable
Loans, net of unearned income	$ 667,034	$ 624,369
Loans at fair value	$ 4,374	3,486
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	$ 326,092	317,187
Loans, net of unearned income	333,656	325,063
Loans, total non-accrual	2,690	3,158
Total loans past due and accruing	$ 2,812	$ 2,991
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	$ 55,029	$ 58,718
Consumer | 80% but less than or equal to 100%
Loans receivable
Loans	4,725	7,120
Consumer | Greater than 100%
Loans receivable
Loans	1,047	1,629
Consumer | Less than 620
Loans receivable
Loans	12,834	13,088
Consumer | ≥ 620 but less than 660
Loans receivable
Loans	14,914	15,431
Consumer | Equal to or greater than 660
Loans receivable
Loans	172,303	172,145
Consumer | Government-guaranteed
Loans receivable
Loans, past due	1,225	1,474
Consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,304	3,275
Consumer | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 3,035	$ 3,127
Consumer | Residential first mortgages
Loans receivable
Number of days past due, non-accrual status	90 days	90 days
Loans at fair value	$ 25	$ 29
Consumer | Residential first mortgages | Less than or equal to 80%
Loans receivable
Loans	43,626	45,849
Consumer | Residential first mortgages | 80% but less than or equal to 100%
Loans receivable
Loans	2,578	3,467
Consumer | Residential first mortgages | Greater than 100%
Loans receivable
Loans	247	324
Consumer | Residential first mortgages | Less than 620
Loans receivable
Loans	2,100	2,744
Consumer | Residential first mortgages | ≥ 620 but less than 660
Loans receivable
Loans	1,932	2,422
Consumer | Residential first mortgages | Equal to or greater than 660
Loans receivable
Loans	$ 42,265	44,279
Consumer | Mortage loans other than FHA-insured loans
Loans receivable
Number of days past due, non-accrual status	60 days
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	$ 11,403	12,869
Consumer | Home equity loans | 80% but less than or equal to 100%
Loans receivable
Loans	2,147	3,653
Consumer | Home equity loans | Greater than 100%
Loans receivable
Loans	800	1,305
Consumer | Home equity loans | Less than 620
Loans receivable
Loans	1,379	1,750
Consumer | Home equity loans | ≥ 620 but less than 660
Loans receivable
Loans	1,081	1,418
Consumer | Home equity loans | Equal to or greater than 660
Loans receivable
Loans	$ 11,976	14,743
Consumer | Credit cards
Loans receivable
Number of days past due, non-accrual status	180 days
Consumer | Credit cards | Less than 620
Loans receivable
Loans	$ 9,079	8,310
Consumer | Credit cards | ≥ 620 but less than 660
Loans receivable
Loans	11,651	11,320
Consumer | Credit cards | Equal to or greater than 660
Loans receivable
Loans	$ 115,577	110,522
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	$ 276	284
Consumer | Installment and other | ≥ 620 but less than 660
Loans receivable
Loans	250	271
Consumer | Installment and other | Equal to or greater than 660
Loans receivable
Loans	$ 2,485	2,601
Consumer | Unsecured Revolving Loans
Loans receivable
Number of days past due, non-accrual status	180 days
Consumer | Commercial market loans
Loans receivable
Number of days past due, non-accrual status	90 days
Consumer | Open-ended consumer loans
Loans receivable
Age modifications limitations in twelve months | re-aging	1
Age modifications limitations in five years | re-aging	2
Consumer | Total GCB and Citi Holdings Consumer
Loans receivable
Loans, current	$ 326,091	317,184
Loans, net of unearned income	333,655	325,060
Loans, total non-accrual	2,690	3,158
Total loans past due and accruing	2,812	2,991
Consumer | Total GCB and Citi Holdings Consumer | Government-guaranteed
Loans receivable
Loans, past due	1,225	1,474
Consumer | Total GCB and Citi Holdings Consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,304	3,275
Consumer | Total GCB and Citi Holdings Consumer | Equal to greater than 90 days past due
Loans receivable
Loans, past due	3,035	3,127
Consumer | Other
Loans receivable
Loans, current	1	3
Loans, net of unearned income	1	3
Loans, total non-accrual	0	0
Total loans 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	0	0
Consumer | Other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	0	0
Consumer | In North America offices
Loans receivable
Loans, current	211,012	213,222
Loans, net of unearned income	216,899	219,491
Loans, total non-accrual	1,650	2,160
Total loans past due and accruing	2,553	2,752
Consumer | In North America offices | Government-guaranteed
Loans receivable
Loans, past due	1,225	1,474
Consumer | In North America offices | 30 to 89 Days Past Due
Loans receivable
Loans, past due	2,336	2,365
Consumer | In North America offices | Equal to greater than 90 days past due
Loans receivable
Loans, past due	2,326	2,430
Consumer | In North America offices | Residential first mortgages
Loans receivable
Loans, current	47,366	50,766
Loans, net of unearned income	49,376	53,133
Loans, total non-accrual	665	848
Total loans past due and accruing	941	1,227
Residential first mortgage loans in process of foreclosure	100	100
Consumer | In North America offices | Residential first mortgages | Government-guaranteed
Loans receivable
Loans, past due	1,225	1,474
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due
Loans receivable
Loans, past due	505	522
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due | Government-guaranteed
Loans receivable
Loans, past due	200	200
Consumer | In North America offices | Residential first mortgages | Equal to greater than 90 days past due
Loans receivable
Loans, past due	280	371
Consumer | In North America offices | Residential first mortgages | Equal to greater than 90 days past due | Government-guaranteed
Loans receivable
Loans, past due	1,000	1,300
Consumer | In North America offices | Home equity loans
Loans receivable
Loans, current	14,268	18,767
Loans, net of unearned income	14,827	19,454
Loans, total non-accrual	750	914
Total loans past due and accruing	0	0
Home equity loans in process of foreclosure,	100	100
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	207	249
Consumer | In North America offices | Home equity loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	352	438
Consumer | In North America offices | Credit cards
Loans receivable
Loans, current	136,588	130,327
Loans, net of unearned income	139,729	133,301
Loans, total non-accrual	0	0
Total loans past due and accruing	1,596	1,509
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	1,528	1,465
Consumer | In North America offices | Credit cards | Equal to greater than 90 days past due
Loans receivable
Loans, past due	1,613	1,509
Consumer | In North America offices | Installment and other
Loans receivable
Loans, current	3,395	4,486
Loans, net of unearned income	3,456	4,630
Loans, total non-accrual	22	70
Total loans past due and accruing	1	2
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	45	106
Consumer | In North America offices | Installment and other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	16	38
Consumer | In North America offices | Commercial market loans
Loans receivable
Loans, current	9,395	8,876
Loans, net of unearned income	9,511	8,973
Loans, total non-accrual	213	328
Total loans past due and accruing	15	14
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	51	23
Consumer | In North America offices | Commercial market loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	65	74
Consumer | In offices outside North America
Loans receivable
Loans, current	115,079	103,962
Loans, net of unearned income	116,756	105,569
Loans, total non-accrual	1,040	998
Total loans past due and accruing	259	239
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	968	910
Consumer | In offices outside North America | Equal to greater than 90 days past due
Loans receivable
Loans, past due	709	697
Consumer | In offices outside North America | Residential first mortgages
Loans receivable
Loans, current	37,062	35,862
Loans, net of unearned income	37,419	36,203
Loans, total non-accrual	400	360
Total loans 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	209	206
Consumer | In offices outside North America | Residential first mortgages | Equal to greater than 90 days past due
Loans receivable
Loans, past due	148	135
Consumer | In offices outside North America | Credit cards
Loans receivable
Loans, current	24,934	22,363
Loans, net of unearned income	25,727	23,055
Loans, total non-accrual	323	258
Total loans past due and accruing	259	239
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	427	368
Consumer | In offices outside North America | Credit cards | Equal to greater than 90 days past due
Loans receivable
Loans, past due	366	324
Consumer | In offices outside North America | Installment and other
Loans receivable
Loans, current	25,634	22,683
Loans, net of unearned income	26,032	23,073
Loans, total non-accrual	157	163
Total loans 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	275	264
Consumer | In offices outside North America | Installment and other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	123	126
Consumer | In offices outside North America | Commercial market loans
Loans receivable
Loans, current	27,449	23,054
Loans, net of unearned income	27,578	23,238
Loans, total non-accrual	160	217
Total loans 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	57	72
Consumer | In offices outside North America | Commercial market loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 72	$ 112