SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) - USD ($) $ in Millions		1 Months Ended	3 Months Ended	12 Months Ended
	Jan. 01, 2023	Jan. 31, 2023	Jun. 30, 2022	Dec. 31, 2024		Dec. 31, 2023		Dec. 31, 2022		Dec. 31, 2021
Loans charge off and interest accrual period
Transfer of investment securities from HTM to AFS	$ 3,300	$ 3,300		$ 0	[1],[2],[3]	$ 3,324	[1],[2],[3]	$ 0	[1],[2],[3]
Fair value transferred to AFS	3,400
Allowance for credit losses on loans (ACLL)				(18,574)		(18,145)		(16,974)		$ (16,455)
Other assets				102,206		95,963
Stockholders’ equity				209,366		206,251		201,838
Deferred tax assets				(29,845)		(29,608)
Decrease to other liabilities				(68,114)		(75,835)
Macroeconomic Uncertainty
Loans charge off and interest accrual period
Decrease in allowance for credit losses			$ (800)
Change in Accounting Method Accounted for as Change in Estimate
Loans charge off and interest accrual period
Decrease in allowance for credit losses			$ 300
Retained earnings
Loans charge off and interest accrual period
Stockholders’ equity				206,294		198,905		194,734		184,948
Accumulated other comprehensive income (loss)
Loans charge off and interest accrual period
Stockholders’ equity				$ (47,852)		(44,800)		(47,062)		(38,765)
Cumulative effect of adoption
Loans charge off and interest accrual period
Allowance for credit losses on loans (ACLL)	352							352
Other assets	40
Deferred tax assets	102
Decrease to other liabilities	39
Cumulative effect of adoption | Retained earnings
Loans charge off and interest accrual period
Stockholders’ equity	290					0	[4]	290	[4]	0	[4]
Cumulative effect of adoption | Accumulated other comprehensive income (loss)
Loans charge off and interest accrual period
Stockholders’ equity	$ 27					$ 0	[5]	$ 27	[5]	$ 0	[5]
Minimum
Loans charge off and interest accrual period
Number of months in sustained period of repayment performance for cash-basis loans to return to an accrual status				6 months
Installment and real estate loans
Loans charge off and interest accrual period
Past due period for consumer loans when interest accrual ceases				90 days
Corporate
Loans charge off and interest accrual period
Period for past due interest or principal payment of corporate loans to be considered doubtful				90 days

[1] In 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.

[2] 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 29 for more information and balances as of December 31, 2024 and 2023.

[3] See Note 2.

[4] See “Accounting Changes” in Note 1.

[5] See Note 1.