LOANS - Consumer Loan Delinquency (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($) payment re-aging	Dec. 31, 2017 USD ($)
Loans receivable
Age modifications limitations in twelve months | re-aging	1
Age modifications limitations in five years | re-aging	2
Loans, net of unearned income	$ 684,196	$ 667,034
Loans at fair value	$ 3,223	4,374
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	$ 323,436	326,092
Loans, net of unearned income	330,487	333,656
Loans, total non-accrual	2,226	2,690
Total loans past due and accruing	$ 2,548	$ 2,797
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	$ 51,844	$ 55,029
Consumer | 80% but less than or equal to 100%
Loans receivable
Loans	3,761	4,725
Consumer | Greater than 100%
Loans receivable
Loans	587	1,047
Consumer | Less than 680
Loans receivable
Loans	40,279	40,541
Consumer | 680 to 760
Loans receivable
Loans	77,963	77,325
Consumer | Greater than 760
Loans receivable
Loans	83,437	82,185
Consumer | Government-guaranteed
Loans receivable
Loans, past due	786	1,225
Consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,264	3,304
Consumer | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 3,001	$ 3,035
Consumer | Residential first mortgages
Loans receivable
Number of days past due, non-accrual status	90 days	90 days
Loans at fair value	$ 20	$ 25
Consumer | Residential first mortgages | Less than or equal to 80%
Loans receivable
Loans	42,379	43,626
Consumer | Residential first mortgages | 80% but less than or equal to 100%
Loans receivable
Loans	2,474	2,578
Consumer | Residential first mortgages | Greater than 100%
Loans receivable
Loans	197	247
Consumer | Residential first mortgages | Less than 680
Loans receivable
Loans	4,530	5,603
Consumer | Residential first mortgages | 680 to 760
Loans receivable
Loans	13,848	14,423
Consumer | Residential first mortgages | Greater than 760
Loans receivable
Loans	$ 26,546	26,271
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	$ 9,465	11,403
Consumer | Home equity loans | 80% but less than or equal to 100%
Loans receivable
Loans	1,287	2,147
Consumer | Home equity loans | Greater than 100%
Loans receivable
Loans	390	800
Consumer | Home equity loans | Less than 680
Loans receivable
Loans	2,438	3,347
Consumer | Home equity loans | 680 to 760
Loans receivable
Loans	4,296	5,439
Consumer | Home equity loans | Greater than 760
Loans receivable
Loans	$ 4,471	5,650
Consumer | Credit cards
Loans receivable
Number of days past due, non-accrual status	180 days
Consumer | Credit cards | Less than 680
Loans receivable
Loans	$ 32,686	30,875
Consumer | Credit cards | 680 to 760
Loans receivable
Loans	58,722	56,443
Consumer | Credit cards | Greater than 760
Loans receivable
Loans	$ 51,299	48,989
Consumer | Installment and other
Loans receivable
Number of days past due, non-accrual status	90 days
Consumer | Installment and other | Less than 680
Loans receivable
Loans	$ 625	716
Consumer | Installment and other | 680 to 760
Loans receivable
Loans	1,097	1,020
Consumer | Installment and other | Greater than 760
Loans receivable
Loans	$ 1,121	1,275
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	$ 323,435	326,091
Loans, net of unearned income	330,486	333,655
Loans, total non-accrual	2,226	2,690
Total loans past due and accruing	2,548	2,797
Consumer | Total GCB and Citi Holdings Consumer | Government-guaranteed
Loans receivable
Loans, past due	786	1,225
Consumer | Total GCB and Citi Holdings Consumer | 30 to 89 Days Past Due
Loans receivable
Loans, past due	3,264	3,304
Consumer | Total GCB and Citi Holdings Consumer | Equal to greater than 90 days past due
Loans receivable
Loans, past due	3,001	3,035
Consumer | Other
Loans receivable
Loans, current	1	1
Loans, net of unearned income	1	1
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	211,250	211,012
Loans, net of unearned income	216,693	216,899
Loans, total non-accrual	1,241	1,650
Total loans past due and accruing	2,313	2,538
Consumer | In North America offices | Government-guaranteed
Loans receivable
Loans, past due	786	1,225
Consumer | In North America offices | 30 to 89 Days Past Due
Loans receivable
Loans, past due	2,331	2,336
Consumer | In North America offices | Equal to greater than 90 days past due
Loans receivable
Loans, past due	2,326	2,326
Consumer | In North America offices | Residential first mortgages
Loans receivable
Loans, current	45,953	47,366
Loans, net of unearned income	47,412	49,376
Loans, total non-accrual	583	665
Total loans past due and accruing	549	941
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	786	1,225
Consumer | In North America offices | Residential first mortgages | 30 to 89 Days Past Due
Loans receivable
Loans, past due	420	505
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	253	280
Consumer | In North America offices | Residential first mortgages | Equal to greater than 90 days past due | Government-guaranteed
Loans receivable
Loans, past due	600	1,000
Consumer | In North America offices | Home equity loans
Loans receivable
Loans, current	11,135	14,268
Loans, net of unearned income	11,543	14,827
Loans, total non-accrual	527	750
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	161	207
Consumer | In North America offices | Home equity loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	247	352
Consumer | In North America offices | Credit cards
Loans receivable
Loans, current	141,106	136,588
Loans, net of unearned income	144,557	139,729
Loans, total non-accrual	0	0
Total loans past due and accruing	1,764	1,596
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	1,687	1,528
Consumer | In North America offices | Credit cards | Equal to greater than 90 days past due
Loans receivable
Loans, past due	1,764	1,613
Consumer | In North America offices | Installment and other
Loans receivable
Loans, current	3,394	3,395
Loans, net of unearned income	3,453	3,456
Loans, total non-accrual	22	22
Total loans past due and accruing	0	1
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	43	45
Consumer | In North America offices | Installment and other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	16	16
Consumer | In North America offices | Commercial market loans
Loans receivable
Loans, current	9,662	9,395
Loans, net of unearned income	9,728	9,511
Loans, total non-accrual	109	213
Total loans past due and accruing	0	0
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	20	51
Consumer | In North America offices | Commercial market loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	46	65
Consumer | In offices outside North America
Loans receivable
Loans, current	112,185	115,079
Loans, net of unearned income	113,793	116,756
Loans, total non-accrual	985	1,040
Total loans past due and accruing	235	259
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	933	968
Consumer | In offices outside North America | Equal to greater than 90 days past due
Loans receivable
Loans, past due	675	709
Consumer | In offices outside North America | Residential first mortgages
Loans receivable
Loans, current	35,624	37,062
Loans, net of unearned income	35,972	37,419
Loans, total non-accrual	383	400
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	203	209
Consumer | In offices outside North America | Residential first mortgages | Equal to greater than 90 days past due
Loans receivable
Loans, past due	145	148
Consumer | In offices outside North America | Credit cards
Loans receivable
Loans, current	24,131	24,934
Loans, net of unearned income	24,926	25,727
Loans, total non-accrual	312	323
Total loans past due and accruing	235	259
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	425	427
Consumer | In offices outside North America | Credit cards | Equal to greater than 90 days past due
Loans receivable
Loans, past due	370	366
Consumer | In offices outside North America | Installment and other
Loans receivable
Loans, current	25,085	25,634
Loans, net of unearned income	25,446	26,032
Loans, total non-accrual	152	157
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	254	275
Consumer | In offices outside North America | Installment and other | Equal to greater than 90 days past due
Loans receivable
Loans, past due	107	123
Consumer | In offices outside North America | Commercial market loans
Loans receivable
Loans, current	27,345	27,449
Loans, net of unearned income	27,449	27,578
Loans, total non-accrual	138	160
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	51	57
Consumer | In offices outside North America | Commercial market loans | Equal to greater than 90 days past due
Loans receivable
Loans, past due	$ 53	$ 72