INVESTMENTS - Debt Securities Held-to-Maturity (Details) - USD ($) $ in Millions	1 Months Ended	9 Months Ended
	Jan. 31, 2023	Sep. 30, 2024		Sep. 30, 2023	[1],[2],[3]	Dec. 31, 2023
Held-to-maturity (HTM) debt securities
Amortized cost, net		$ 248,274				$ 254,247
Gross unrealized gains		146				91
Gross unrealized losses		14,110				19,337
Fair value		234,310				235,001
Allowance for credit losses on HTM debt securities		141				95
Transfer of investment securities from HTM to AFS, amortized cost	$ 3,300	0	[1],[2],[3]	$ 3,324
Transfer of securities from HTM to AFS, unrealized gain	$ 100
Mortgage-backed securities
Held-to-maturity (HTM) debt securities
Amortized cost, net		75,596				81,033
Gross unrealized gains		15				9
Gross unrealized losses		7,223				8,759
Fair value		68,388				72,283
U.S. government-sponsored agency guaranteed
Held-to-maturity (HTM) debt securities
Amortized cost, net		74,315				79,689
Gross unrealized gains		9				7
Gross unrealized losses		7,113				8,603
Fair value		67,211				71,093
Mortgage-backed securities - Non-U.S. residential
Held-to-maturity (HTM) debt securities
Amortized cost, net		139				198
Gross unrealized gains		1				0
Gross unrealized losses		0				0
Fair value		140				198
Commercial Mortgage-Backed Securities
Held-to-maturity (HTM) debt securities
Amortized cost, net		1,142				1,146
Gross unrealized gains		5				2
Gross unrealized losses		110				156
Fair value		1,037				992
U.S. treasury securities
Held-to-maturity (HTM) debt securities
Amortized cost, net		131,350				131,776
Gross unrealized gains		0				0
Gross unrealized losses		6,371				9,908
Fair value		124,979				121,868
State and municipal
Held-to-maturity (HTM) debt securities
Amortized cost, net		8,958				9,182
Gross unrealized gains		67				73
Gross unrealized losses		457				477
Fair value		8,568				8,778
Foreign government
Held-to-maturity (HTM) debt securities
Amortized cost, net		1,506				2,210
Gross unrealized gains		11				0
Gross unrealized losses		7				58
Fair value		1,510				2,152
Asset-backed securities
Held-to-maturity (HTM) debt securities
Amortized cost, net		30,864				30,046
Gross unrealized gains		53				9
Gross unrealized losses		52				135
Fair value		$ 30,865				$ 29,920

[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 26 for more information and balances as of September 30, 2024.

[3] See Note 2.