INVESTMENTS - Overview (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		9 Months Ended
	Jan. 31, 2023	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2023		Sep. 30, 2022		Dec. 31, 2022
Investments
Total investments		$ 508,998		$ 508,998				$ 526,582
Accrued interest receivable		2,000		2,000				2,000
Interest and dividends on investments
Taxable interest		4,547	$ 2,714	12,831		$ 7,001
Interest exempt from U.S. federal income tax		83	220	252		263
Dividend income		89	59	231		149
Total interest and dividend income on investments		4,719	2,993	13,314		7,413
Gross realized investments losses, excluding losses from other-than-temporary impairment
Gross realized investment gains		83	102	262		282
Gross realized investment losses		(53)	(50)	(111)		(208)
Net realized gains on sales of investments		30	$ 52	151		74
Debt securities AFS
Allowance for credit losses		5		5				3
Fair value		241,783		241,783				249,679
Transfer of investment securities from HTM to AFS, amortized cost	$ 3,300			3,324	[1],[2],[3]	$ 0	[1],[2],[3]
Transfer of securities from HTM to AFS, unrealized gain	$ 100
Cumulative basis adjustments		(171)		(171)
Gross unrealized gains excluding the effect of unallocated portfolio cumulative basis adjustments		2		2
Gross unrealized losses excluding the effect of unallocated portfolio cumulative basis adjustments		1,500		1,500
Debt securities available-for-sale (AFS)
Investments
Total investments		241,783		241,783				249,679
Debt securities held-to-maturity (HTM)
Investments
Total investments		259,456		259,456				268,863
Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Debt securities AFS
Amortized cost		20,750		20,750				12,009
Gross unrealized gains		23		23				8
Gross unrealized losses		958		958				755
Allowance for credit losses		0		0				0
Fair value		19,815		19,815				11,262
Residential mortgages
Debt securities AFS
Amortized cost		310		310				488
Gross unrealized gains		0		0				0
Gross unrealized losses		3		3				3
Allowance for credit losses		0		0				0
Fair value		307		307				485
Commercial
Debt securities AFS
Amortized cost		1		1				2
Gross unrealized gains		0		0				0
Gross unrealized losses		0		0				0
Allowance for credit losses		0		0				0
Fair value		1		1				2
Mortgage-backed securities
Debt securities AFS
Amortized cost		21,061		21,061				12,499
Gross unrealized gains		23		23				8
Gross unrealized losses		961		961				758
Allowance for credit losses		0		0				0
Fair value		20,123		20,123				11,749
U.S. Treasury
Debt securities AFS
Amortized cost		83,029		83,029				94,732
Gross unrealized gains		21		21				50
Gross unrealized losses		1,782		1,782				2,492
Allowance for credit losses		0		0				0
Fair value		81,268		81,268				92,290
Agency obligations
Debt securities AFS
Amortized cost		0		0				0
Gross unrealized gains		0		0				0
Gross unrealized losses		0		0				0
Allowance for credit losses		0		0				0
Fair value		0		0				0
U.S. Treasury and federal agency securities
Debt securities AFS
Amortized cost		83,029		83,029				94,732
Gross unrealized gains		21		21				50
Gross unrealized losses		1,782		1,782				2,492
Allowance for credit losses		0		0				0
Fair value		81,268		81,268				92,290
State and municipal
Debt securities AFS
Amortized cost		2,204		2,204				2,363
Gross unrealized gains		6		6				19
Gross unrealized losses		179		179				159
Allowance for credit losses		0		0				0
Fair value		2,031		2,031				2,223
Foreign government
Debt securities AFS
Amortized cost		127,245		127,245				135,648
Gross unrealized gains		234		234				569
Gross unrealized losses		2,083		2,083				2,940
Allowance for credit losses		0		0				0
Fair value		125,396		125,396				133,277
Corporate
Debt securities AFS
Amortized cost		5,653		5,653				5,146
Gross unrealized gains		13		13				19
Gross unrealized losses		268		268				246
Allowance for credit losses		5		5				3
Fair value		5,393		5,393				4,916
Asset-backed securities
Debt securities AFS
Amortized cost		681		681				1,022
Gross unrealized gains		3		3				12
Gross unrealized losses		2		2				4
Allowance for credit losses		0		0				0
Fair value		682		682				1,030
Other debt securities
Debt securities AFS
Amortized cost		6,890		6,890				4,198
Gross unrealized gains		2		2				1
Gross unrealized losses		2		2				5
Allowance for credit losses		0		0				0
Fair value		6,890		6,890				4,194
Debt securities AFS
Debt securities AFS
Amortized cost		246,763		246,763				255,608
Gross unrealized gains		302		302				678
Gross unrealized losses		5,277		5,277				6,604
Allowance for credit losses		5		5				3
Fair value		241,783		241,783				249,679
Cumulative basis adjustments		(500)		(500)
Fair value | Marketable equity securities carried at fair value
Investments
Total investments		283		283				429
Fair value | Non-marketable equity securities
Investments
Total investments		455		455				466
Fair value | Non-marketable securities measured using measurement alternative
Investments
Total investments		1,621		1,621				1,676
Non-marketable equity securities measured using the measurement alternative		24		24				27
Carried at cost | Non-marketable equity securities
Investments
Total investments		$ 5,400		$ 5,400				$ 5,469

[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 25 for more information and balances as of September 30, 2023.

[2] See Note 2 for further information on significant disposals.

[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.