REGULATORY CAPITAL - Regulatory Capital Compliance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
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	6.00%
Total Capital ratio, stated minimum	8.00%
Tier 1 Leverage ratio, stated minimum	4.00%
Supplementary leverage ratio, stated minimum	3.00%
Dividends received from Citibank N.A.	$ 17,300	$ 8,300
Citigroup Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital	137,798	139,252
Tier 1 Capital	155,805	158,122
Quarterly Adjusted Average Total Assets	1,957,039	1,896,959
Total Leverage Exposure	$ 2,507,891	$ 2,465,641
Common Equity Tier 1 Capital ratio	11.81%	11.86%
Tier 1 Capital ratio, well capitalized minimum	6.00%
Tier 1 Capital ratio	13.36%	13.46%
Total Capital ratio, well capitalized minimum	10.00%
Total Capital ratio	15.97%	16.18%
Tier 1 Leverage ratio	7.96%	8.34%
Supplementary leverage ratio	6.21%	6.41%
Citibank, N.A.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital	$ 130,791	$ 129,091
Tier 1 Capital	132,918	131,215
Quarterly Adjusted Average Total Assets	1,459,851	1,398,875
Total Leverage Exposure	$ 1,951,701	$ 1,914,663
Common Equity Tier 1 Capital ratio	12.82%	12.50%
Common Equity Tier 1 Capital ratio, well capitalized minimum	6.50%
Tier 1 Capital ratio, well capitalized minimum	8.00%
Tier 1 Capital ratio	13.03%	12.70%
Total Capital ratio, well capitalized minimum	10.00%
Total Capital ratio	15.43%	15.02%
Tier 1 Leverage ratio	9.10%	9.38%
Tier 1 Leverage ratio, well capitalized minimum	5.00%
Supplementary leverage ratio	6.81%	6.85%
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)	$ 193,682	$ 195,440
Total Risk-Weighted Assets	1,166,523	1,174,448
Standardized Approach | Citibank, N.A.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (Tier 1 Capital and Tier 2 Capital)	157,324	155,154
Total Risk-Weighted Assets	1,019,916	1,032,809
Advanced Approach | Citigroup Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (Tier 1 Capital and Tier 2 Capital)	181,337	183,144
Total Risk-Weighted Assets	1,135,553	1,131,933
Advanced Approach | Citibank, N.A.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (Tier 1 Capital and Tier 2 Capital)	145,989	144,358
Total Risk-Weighted Assets	$ 932,432	$ 926,229