REGULATORY CAPITAL - Regulatory Capital Compliance (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
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.	$ 8,300	$ 7,500
Citigroup Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital	139,252
Tier 1 Capital	158,122
Quarterly Adjusted Average Total Assets	1,896,959
Total Leverage Exposure	$ 2,465,641
Common Equity Tier 1 Capital ratio	11.86%
Tier 1 Capital ratio, well capitalized minimum	6.00%
Tier 1 Capital ratio	13.46%
Total Capital ratio, well capitalized minimum	10.00%
Total Capital ratio	16.18%
Tier 1 Leverage ratio	8.34%
Supplementary leverage ratio	6.41%
Citibank, N.A.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 Capital	$ 129,217
Tier 1 Capital	131,341
Quarterly Adjusted Average Total Assets	1,399,029
Total Leverage Exposure	$ 1,914,817
Common Equity Tier 1 Capital ratio	12.54%
Common Equity Tier 1 Capital ratio, well capitalized minimum	6.50%
Tier 1 Capital ratio, well capitalized minimum	8.00%
Tier 1 Capital ratio	12.75%
Total Capital ratio, well capitalized minimum	10.00%
Total Capital ratio	15.07%
Tier 1 Leverage ratio	9.39%
Tier 1 Leverage ratio, well capitalized minimum	5.00%
Supplementary leverage ratio	6.86%
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)	$ 195,440
Total Risk-Weighted Assets	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)	155,280
Total Risk-Weighted Assets	1,030,514
Advanced Approach \| Citigroup Inc.
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital (Tier 1 Capital and Tier 2 Capital)	183,144
Total Risk-Weighted Assets	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)	144,485
Total Risk-Weighted Assets	$ 927,931

Amount of leverage assets as calculated in accordance with U.S. Federal regulatory capital standards.

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Dividends that Citigroup received from Citibank during the current period.

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Supplementary Leverage Ratio Required to Be Well Capitalized To Risk Weighted Assets

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Supplementary Leverage Ratio, Stated Minimum

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Tier 1 common capital, as defined by Supervisory Capital Assessment Program by U.S. banking regulators.

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Tier 1 common capital divided by risk weighted assets, as by Supervisory Capital Assessment Program by U.S. banking regulators.

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Tier One Common Capital Required for Capital Adequacy to Risk Weighted Assets

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Tier One Common Equity Capital Required to Be Well Capitalized To Risk Weighted Assets

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Total Leverage Exposure

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The amount of total capital as defined in the regulations.

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The minimum Total risk based capital ratio (total capital divided by risk weighted assets) required for capital adequacy purposes under the regulatory framework for prompt corrective action.

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The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

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Total Capital divided by risk weighted assets as defined in the regulations.

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Amount of risk-weighted assets as calculated in accordance with U.S. Federal regulatory capital standards.

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The minimum amount of Tier 1 Leverage Capital for capital adequacy purposes divided by average assets as defined in the regulations.

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The amount of Tier 1 Leverage Capital required to be categorized as well capitalized divided by average assets as defined in the regulatory framework for prompt corrective action.

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Tier 1 Leverage Capital divided by average assets as defined in the regulations.

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Tier 1 Risk Based Capital as defined in the regulations.

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The Tier 1 capital ratio (Tier 1 capital divided by risk weighted assets) required to be categorized as "well capitalized" under the regulatory framework for prompt corrective action.

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Tier 1 capital divided by risk weighted assets as defined by regulations.

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