INVESTMENTS - Overview (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended
	Jan. 01, 2023	Jan. 31, 2023	Mar. 31, 2024		Mar. 31, 2023		Dec. 31, 2023
Investments
Total investments			$ 515,183.0				$ 519,085.0
Accrued interest receivable			2,000.0				2,000.0
Interest and dividends on investments
Taxable interest			4,691.0		$ 4,000.0
Interest exempt from U.S. federal income tax			80.0		85.0
Dividend income			78.0		59.0
Total interest and dividend income on investments			4,849.0		4,144.0
Gross realized investments losses, excluding losses from other-than-temporary impairment
Gross realized investment gains			141.0		88.0
Gross realized investment losses			(26.0)		(16.0)
Net realized gains on sales of investments			115.0		72.0
Debt securities AFS
Allowance for credit losses			9.0				8.0
Fair value			254,898.0				256,936.0
Transfer of investment securities from HTM to AFS, amortized cost	$ 3,300.0	$ 3,300.0	0.0	[1],[2],[3]	$ 3,324.0	[1],[2],[3]
Transfer of securities from HTM to AFS, unrealized gain		$ 100.0
Unallocated portfolio layer cumulative basis adjustments			(113.0)				93.0
Gross unrealized gains excluding the effect of unallocated portfolio cumulative basis adjustments			140.0
Gross unrealized losses excluding the effect of unallocated portfolio cumulative basis adjustments			(860.0)
Debt securities available-for-sale (AFS)
Investments
Total investments			254,898.0				256,936.0
Debt securities held-to-maturity (HTM)
Investments
Total investments			252,459.0				254,247.0
U.S. government-sponsored agency guaranteed
Debt securities AFS
Amortized cost			32,547.0				30,279.0
Gross unrealized gains			134.0				170.0
Gross unrealized losses			739.0				734.0
Allowance for credit losses			0.0				0.0
Fair value			31,942.0				29,715.0
Residential
Debt securities AFS
Amortized cost			530.0				426.0
Gross unrealized gains			0.0				0.0
Gross unrealized losses			3.0				3.0
Allowance for credit losses			0.0				0.0
Fair value			527.0				423.0
Commercial Mortgage-Backed Securities
Debt securities AFS
Amortized cost			1.0				1.0
Gross unrealized gains			0.0				0.0
Gross unrealized losses			0.0				0.0
Allowance for credit losses			0.0				0.0
Fair value			1.0				1.0
Mortgage-backed securities
Debt securities AFS
Amortized cost			33,078.0				30,706.0
Gross unrealized gains			134.0				170.0
Gross unrealized losses			742.0				737.0
Allowance for credit losses			0.0				0.0
Fair value			32,470.0				30,139.0
U.S. Treasury
Debt securities AFS
Amortized cost			76,206.0				81,684.0
Gross unrealized gains			8.0				59.0
Gross unrealized losses			1,181.0				1,382.0
Allowance for credit losses			0.0				0.0
Fair value			75,033.0				80,361.0
U.S. Treasury and federal agency securities
Debt securities AFS
Amortized cost			76,206.0				81,684.0
Gross unrealized gains			8.0				59.0
Gross unrealized losses			1,181.0				1,382.0
Allowance for credit losses			0.0				0.0
Fair value			75,033.0				80,361.0
State and municipal
Debt securities AFS
Amortized cost			2,064.0				2,204.0
Gross unrealized gains			21.0				18.0
Gross unrealized losses			97.0				91.0
Allowance for credit losses			0.0				0.0
Fair value			1,988.0				2,131.0
Foreign government
Debt securities AFS
Amortized cost			134,579.0				132,045.0
Gross unrealized gains			444.0				528.0
Gross unrealized losses			1,325.0				1,375.0
Allowance for credit losses			0.0				0.0
Fair value			133,698.0				131,198.0
Corporate
Debt securities AFS
Amortized cost			5,203.0				5,610.0
Gross unrealized gains			19.0				18.0
Gross unrealized losses			192.0				208.0
Allowance for credit losses			9.0				8.0
Fair value			5,021.0				5,412.0
Asset-backed securities
Debt securities AFS
Amortized cost			930.0				921.0
Gross unrealized gains			10.0				17.0
Gross unrealized losses			1.0				0.0
Allowance for credit losses			0.0				0.0
Fair value			939.0				938.0
Other debt securities
Debt securities AFS
Amortized cost			5,746.0				6,754.0
Gross unrealized gains			4.0				4.0
Gross unrealized losses			1.0				1.0
Allowance for credit losses			0.0				0.0
Fair value			5,749.0				6,757.0
Debt securities AFS
Debt securities AFS
Amortized cost			257,806.0				259,924.0
Gross unrealized gains			640.0				814.0
Gross unrealized losses			3,539.0				3,794.0
Allowance for credit losses			9.0				8.0
Fair value			254,898.0				256,936.0
Unallocated portfolio layer cumulative basis adjustments			(100.0)
Fair value | Marketable equity securities carried at fair value
Investments
Total investments			287.0				258.0
Fair value | Non-marketable equity securities
Investments
Total investments			515.0				508.0
Fair value | Non-marketable securities measured using measurement alternative
Investments
Total investments			1,706.0				1,639.0
Non-marketable equity securities measured using the measurement alternative			27.0				25.0
Carried at cost | Non-marketable equity securities
Investments
Total investments			$ 5,318.0				$ 5,497.0

[1] Operating and finance lease right-of-use assets and lease liabilities represent non-cash investing and financing activities, respectively, and are not included in the non-cash investing activities presented here. See Note 26 for more information and balances as of March 31, 2024.

[2] See Note 2.

[3] n January 2023, Citi adopted ASU 2022-01. Upon adoption, Citi transferred $3.3 billion of mortgage-backed securities from HTM classification to AFS classification as allowed under the ASU. At the time of transfer, the securities were in an unrealized gain position of $0.1 billion, which was recorded in AOCI upon transfer.