BASIS OF PRESENTATION, UPDATED ACCOUNTING POLICIES AND ACCOUNTING CHANGES - Accounting Changes (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		Dec. 31, 2022
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses			$ (18,296)		$ (17,169)		$ (18,145)		$ (16,974)
Other assets, at fair value			97,701				95,963
Stockholders’ equity			207,398		209,019		206,251
Other liabilities			(71,492)				(75,835)
Transfer of investment securities from HTM to AFS, amortized cost	$ 3,300	$ 3,300	0	[1],[2],[3]	3,324	[1],[2],[3]
Transfer of investment securities from HTM to AFS	3,400
Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Allowance for credit losses	352								352
Other assets, at fair value	44
Deferred tax assets	106
Other liabilities	39
Retained earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders’ equity			200,956		198,353		198,905		194,734
Retained earnings | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders’ equity	290						0	[4]	290	[4]
Accumulated other comprehensive income (loss)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders’ equity			$ (45,729)		$ (45,441)		(44,800)		(47,062)
Accumulated other comprehensive income (loss) | Cumulative Effect, Period of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Stockholders’ equity	$ 27						$ 0	[4]	$ 27	[4]

[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.

[4] See Note 1 for additional details.