REGULATORY CAPITAL - Regulatory Capital Compliance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital ratio, stated minimum	4.50%
Tier 1 Capital ratio, stated minimum	0.060
Total Capital ratio, stated minimum	0.080
Tier 1 Leverage ratio, stated minimum	0.040
Supplementary leverage ratio, stated minimum	3.00%
Dividends received from Citibank N.A.	$ 2,300	$ 17,300
Citigroup Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital	147,274	137,798
Tier 1 Capital	167,053	155,805
Quarterly Adjusted Average Total Assets	2,265,615	1,957,039
Total Leverage Exposure	$ 2,386,881	$ 2,513,702
Common Equity Tier 1 Capital ratio	11.73%	11.79%
Tier 1 Capital ratio, well capitalized minimum	0.060
Tier 1 Capital ratio	0.1331	0.1333
Total Capital ratio, well capitalized minimum	0.100
Total Capital ratio	0.1561	0.1587
Tier 1 Leverage ratio	0.0737	0.0796
Supplementary leverage ratio	0.0700	0.0620
Citibank, N.A.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital	$ 142,884	$ 130,720
Tier 1 Capital	144,992	132,847
Quarterly Adjusted Average Total Assets	1,680,056	1,459,780
Total Leverage Exposure	$ 2,167,969	$ 1,958,173
Common Equity Tier 1 Capital ratio	13.87%	12.78%
Common Equity Tier 1 Capital ratio, well capitalized minimum	6.50%
Tier 1 Capital ratio, well capitalized minimum	0.080
Tier 1 Capital ratio	0.1408	0.1299
Total Capital ratio, well capitalized minimum	0.100
Total Capital ratio	0.1594	0.1538
Tier 1 Leverage ratio	0.0863	0.0910
Tier 1 Leverage ratio, well capitalized minimum	0.050
Supplementary leverage ratio	0.0669	0.0678
Supplementary leverage ratio, well capitalized minimum	6.00%
Standardized Approach | Citigroup Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (Tier 1 Capital and Tier 2 Capital)	$ 204,849	$ 193,711
Total Risk-Weighted Assets	1,221,576	1,168,848
Standardized Approach | Citibank, N.A.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (Tier 1 Capital and Tier 2 Capital)	169,235	157,253
Total Risk-Weighted Assets	1,030,081	1,022,607
Advanced Approach | Citigroup Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (Tier 1 Capital and Tier 2 Capital)	195,959	181,337
Total Risk-Weighted Assets	1,255,284	1,142,804
Advanced Approach | Citibank, N.A.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (Tier 1 Capital and Tier 2 Capital)	161,294	145,918
Total Risk-Weighted Assets	$ 1,012,129	$ 938,735