INVESTMENTS - Debt Securities Held-to-Maturity (Details) - USD ($) $ in Millions			1 Months Ended	12 Months Ended
		Jan. 01, 2023	Jan. 31, 2023	Dec. 31, 2023		Dec. 31, 2022		Dec. 31, 2021
Debt Securities Held-to-maturity
Total amortized cost				$ 254,247		$ 268,863
Gross unrecognized gains				91		101
Gross unrecognized losses				19,337		25,316
Fair value				235,001		243,648
Fair value of securities transferred from AFS to HTM	[1],[2],[3]			0		21,688		$ 0
Transfer of investment securities from HTM to AFS		$ 3,300	$ 3,300	3,324	[1],[2],[3]	0	[1],[2],[3]	$ 0	[1],[2],[3]
Transfer of securities from HTM to AFS, unrealized gain			$ 100
U.S. government-sponsored agency guaranteed
Debt Securities Held-to-maturity
Total amortized cost				79,689		90,063
Gross unrecognized gains				7		58
Gross unrecognized losses				8,603		10,033
Fair value				71,093		80,088
Mortgage-backed securities - Non-U.S. residential
Debt Securities Held-to-maturity
Total amortized cost				198		445
Gross unrecognized gains				0		0
Gross unrecognized losses				0		0
Fair value				198		445
Commercial
Debt Securities Held-to-maturity
Total amortized cost				1,146		1,114
Gross unrecognized gains				2		5
Gross unrecognized losses				156		1
Fair value				992		1,118
Mortgage-backed securities
Debt Securities Held-to-maturity
Total amortized cost				81,033		91,622
Gross unrecognized gains				9		63
Gross unrecognized losses				8,759		10,034
Fair value				72,283		81,651
US Treasury securities
Debt Securities Held-to-maturity
Total amortized cost				131,776		134,961
Gross unrecognized gains				0		0
Gross unrecognized losses				9,908		13,722
Fair value				121,868		121,239
State and municipal
Debt Securities Held-to-maturity
Total amortized cost				9,182		9,237
Gross unrecognized gains				73		34
Gross unrecognized losses				477		764
Fair value				8,778		8,507
Foreign government
Debt Securities Held-to-maturity
Total amortized cost				2,210		2,075
Gross unrecognized gains				0		0
Gross unrecognized losses				58		93
Fair value				2,152		1,982
Asset-backed securities
Debt Securities Held-to-maturity
Total amortized cost				30,046		30,968
Gross unrecognized gains				9		4
Gross unrecognized losses				135		703
Fair value				$ 29,920		$ 30,269

[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.