INVESTMENTS - Overview (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended
	Jan. 31, 2023	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023		Jun. 30, 2022		Dec. 31, 2022
Investments
Total investments		$ 507,145		$ 507,145				$ 526,582
Accrued interest receivable		2,000		2,000				2,000
Interest and dividends on investments
Taxable interest		4,284	$ 2,274	8,284		$ 4,287
Interest exempt from U.S. federal income tax		84	38	169		43
Dividend income		83	58	142		90
Total interest and dividend income on investments		4,451	2,370	8,595		4,420
Gross realized investments losses, excluding losses from other-than-temporary impairment
Gross realized investment gains		91	27	179		180
Gross realized investment losses		(42)	(85)	(58)		(158)
Net realized gains (losses) on sales of investments		49	$ (58)	121		22
Debt securities AFS
Allowance for credit losses		5		5				3
Fair value		237,334		237,334				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		(200)		(200)
Gross unrealized gains excluding the effect of unallocated portfolio cumulative basis adjustments		32		32
Gross unrealized losses excluding the effect of unallocated portfolio cumulative basis adjustments		848		848
Debt securities available-for-sale (AFS)
Investments
Total investments		237,334		237,334				249,679
Debt securities held-to-maturity (HTM)
Investments
Total investments		262,066		262,066				268,863
Mortgage-backed securities - U.S. government-sponsored agency guaranteed
Debt securities AFS
Amortized cost		15,578		15,578				12,009
Gross unrealized gains		82		82				8
Gross unrealized losses		697		697				755
Allowance for credit losses		0		0				0
Fair value		14,963		14,963				11,262
Residential mortgages
Debt securities AFS
Amortized cost		340		340				488
Gross unrealized gains		0		0				0
Gross unrealized losses		3		3				3
Allowance for credit losses		0		0				0
Fair value		337		337				485
Commercial
Debt securities AFS
Amortized cost		2		2				2
Gross unrealized gains		0		0				0
Gross unrealized losses		0		0				0
Allowance for credit losses		0		0				0
Fair value		2		2				2
Mortgage-backed securities
Debt securities AFS
Amortized cost		15,920		15,920				12,499
Gross unrealized gains		82		82				8
Gross unrealized losses		700		700				758
Allowance for credit losses		0		0				0
Fair value		15,302		15,302				11,749
U.S. Treasury
Debt securities AFS
Amortized cost		84,250		84,250				94,732
Gross unrealized gains		23		23				50
Gross unrealized losses		2,030		2,030				2,492
Allowance for credit losses		0		0				0
Fair value		82,243		82,243				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		84,250		84,250				94,732
Gross unrealized gains		23		23				50
Gross unrealized losses		2,030		2,030				2,492
Allowance for credit losses		0		0				0
Fair value		82,243		82,243				92,290
State and municipal
Debt securities AFS
Amortized cost		2,305		2,305				2,363
Gross unrealized gains		24		24				19
Gross unrealized losses		127		127				159
Allowance for credit losses		0		0				0
Fair value		2,202		2,202				2,223
Foreign government
Debt securities AFS
Amortized cost		126,967		126,967				135,648
Gross unrealized gains		356		356				569
Gross unrealized losses		2,053		2,053				2,940
Allowance for credit losses		0		0				0
Fair value		125,270		125,270				133,277
Corporate
Debt securities AFS
Amortized cost		5,596		5,596				5,146
Gross unrealized gains		13		13				19
Gross unrealized losses		277		277				246
Allowance for credit losses		5		5				3
Fair value		5,327		5,327				4,916
Asset-backed securities
Debt securities AFS
Amortized cost		842		842				1,022
Gross unrealized gains		7		7				12
Gross unrealized losses		3		3				4
Allowance for credit losses		0		0				0
Fair value		846		846				1,030
Other debt securities
Debt securities AFS
Amortized cost		6,145		6,145				4,198
Gross unrealized gains		2		2				1
Gross unrealized losses		3		3				5
Allowance for credit losses		0		0				0
Fair value		6,144		6,144				4,194
Debt securities AFS
Debt securities AFS
Amortized cost		242,025		242,025				255,608
Gross unrealized gains		507		507				678
Gross unrealized losses		5,193		5,193				6,604
Allowance for credit losses		5		5				3
Fair value		237,334		237,334				249,679
Fair value | Marketable equity securities carried at fair value
Investments
Total investments		296		296				429
Fair value | Non-marketable equity securities
Investments
Total investments		433		433				466
Fair value | Non-marketable securities measured using measurement alternative
Investments
Total investments		1,618		1,618				1,676
Non-marketable equity securities measured using the measurement alternative		24		24				27
Carried at cost | Non-marketable equity securities
Investments
Total investments		$ 5,398		$ 5,398				$ 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 June 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. See Note 1.