SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) $ in Millions		1 Months Ended	12 Months Ended
	Jan. 01, 2023 USD ($)	Jan. 31, 2023 USD ($)	Dec. 31, 2023 USD ($) payment		Dec. 31, 2022 USD ($)	[1],[2],[3]	Dec. 31, 2021 USD ($)	[1],[2],[3]
Loans charge off and interest accrual period
Transfer of investment securities from HTM to AFS | $	$ 3,300	$ 3,300	$ 3,324	[1],[2],[3]	$ 0		$ 0
Fair value transferred to AFS | $	$ 3,400
Minimum
Loans charge off and interest accrual period
Minimum number of payments on a modified loan to return to accrual status			1
Number of months in sustained period of repayment performance for cash-basis loans to return to an accrual status			6 months
Maximum
Loans charge off and interest accrual period
Minimum number of payments on a modified loan to return to accrual status			6
Installment and real estate loans
Loans charge off and interest accrual period
Past due period for consumer loans when interest accrual ceases			90 days
Credit cards and unsecured revolving loans
Loans charge off and interest accrual period
Past due period for consumer loans when interest accrual ceases			180 days
Period for consumer loans charged-off			180 days
Home equity loans | New OCC guidance
Loans charge off and interest accrual period
Number of days past due, non-accrual status			90 days
Consumer | Minimum
Loans charge off and interest accrual period
Minimum number of payments required for modification of loans			1
Consumer | Maximum
Loans charge off and interest accrual period
Minimum number of payments required for modification of loans			3
Open-ended consumer loans
Loans charge off and interest accrual period
Minimum number of payments required for modification of loans			3
Unsecured installment loans
Loans charge off and interest accrual period
Period for consumer loans charged-off			120 days
Mortgage and real estate
Loans charge off and interest accrual period
Maximum past due period of contractual payments to write down secured loans to net collateral values			60 days
Period for consumer loans charged-off			180 days
Real estate-secured loans
Loans charge off and interest accrual period
Past due period of contractual payments to charge-off, if no payments in six months and if a decision has been made not to foreclose on the loans			180 days
Unsecured loans in bankruptcy
Loans charge off and interest accrual period
Maximum period from receiving bankruptcy court filing notification to charge-off			60 days
Real estate-secured loans in bankruptcy
Loans charge off and interest accrual period
Maximum past due period of contractual payments to write down secured loans to net collateral values			60 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
Time period for reversing accrued interest on impaired corporate loans			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, 2023 and 2022.

[3] See Note 2.