INCOME TAXES - Tax Credit Carryforward and Expiration Dates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
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	$ 3,000
Net DFA	$ 22,481	$ 46,668
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	$ 52,000
Decrease in deferred tax assets due to FTC carryforwards	6,600
Decrease in deferred tax assets	24,200
Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	13,300	14,200
Operating loss carryforward	2,000	2,100
Net DFA	2,600	2,100
U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	1,400	900
Operating loss carryforward	11,000	10,000
U.S. federal
Operating Loss and Tax Carryforwards
Net DFA	16,100	40,500
Foreign tax credits (FTCs)	7,600	14,200
Gross foreign tax credits	13,300
2018 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	400	2,700
2019 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	1,300	1,300
2020 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	3,200	3,100
2021 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	2,000	1,900
2022 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	3,400	3,300
2023 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	400	500
2025 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	1,400	1,400
2027 | Foreign
Operating Loss and Tax Carryforwards
Tax credit carryforward	1,200	0
2027 | U.S. federal
Operating Loss and Tax Carryforwards
Operating loss carryforward	200	200
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	200	0
Operating loss carryforward	100	0
2033 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	300	300
Operating loss carryforward	1,600	1,700
2034 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	200	200
Operating loss carryforward	2,300	2,300
2034 | New York state tax authority
Operating Loss and Tax Carryforwards
Operating loss carryforward	13,600	13,000
2034 | New York City tax authority
Operating Loss and Tax Carryforwards
Operating loss carryforward	13,100	12,200
2035 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	200	200
Operating loss carryforward	3,300	3,200
2036 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	200	200
Operating loss carryforward	2,100	2,200
2037 | U.S. federal
Operating Loss and Tax Carryforwards
Tax credit carryforward	300	0
Operating loss carryforward	$ 1,000	$ 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%