INCOME TAXES - Tax Credit Carryforward and Expiration Dates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating Loss and Tax Carryforwards
Foreign tax credits related to companies filing U.S. federal tax separate from Citigroup's consolidated U.S. federal tax return expected to be utilized in consolidated return	$ 2,900
Net DFA	$ 22,903	$ 22,481
U.S. Federal and New York State and City NOL carryforward period	20 years
U.S. foreign tax credit carryforward period (in years)	10 years
Limit on utilization of foreign tax credit carryforwards (as percent)	21.00%
Domestic losses allowed to be reclassified as foreign source income	$ 47,000
Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	12,800	13,300
Operating loss carryforward	2,000	2,000
Net DFA	2,200	2,600
U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	1,000	1,400
Operating loss carryforward	12,500	11,000
U.S. federal
Operating Loss and Tax Carryforwards
Net DFA	17,100	16,100
Foreign tax credits (FTCs)	6,800	7,600
Gross foreign tax credits	12,800
2018 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	0	400
2019 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	900	1,300
2020 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	2,600	3,200
2021 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	1,800	2,000
2022 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	3,300	3,400
2023 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	400	400
2025 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	1,400	1,400
2027 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	1,100	1,200
2027 | U.S. federal
Operating Loss and Tax Carryforwards
Operating loss carryforward	200	200
2028 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	1,300	0
2028 | U.S. federal
Operating Loss and Tax Carryforwards
Operating loss carryforward	100	100
2030 | U.S. federal
Operating Loss and Tax Carryforwards
Operating loss carryforward	300	300
2032 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	0	200
Operating loss carryforward	100	100
2033 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	0	300
Operating loss carryforward	1,600	1,600
2034 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	0	200
Operating loss carryforward	2,100	2,300
2034 | New York state tax authority
Operating Loss and Tax Carryforwards
Operating loss carryforward	11,700	13,600
2034 | New York City tax authority
Operating Loss and Tax Carryforwards
Operating loss carryforward	11,500	13,100
2035 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	0	200
Operating loss carryforward	3,300	3,300
2036 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	100	200
Operating loss carryforward	2,100	2,100
2037 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	400	300
Operating loss carryforward	1,000	1,000
2038 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	500	0
Unlimited Carry-Forward Period | U.S. federal
Operating Loss and Tax Carryforwards
Operating loss carryforward	$ 1,700	$ 0
Minimum
Operating Loss and Tax Carryforwards
Limit on domestic losses to be reclassified as foreign source income (as a percent)	50.00%
Maximum
Operating Loss and Tax Carryforwards
Limit on domestic losses to be reclassified as foreign source income (as a percent)	100.00%