INVESTMENTS - Overview (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
Investment Holdings [Line Items]
Investments				$ 519,085		$ 526,582
Accrued interest receivable				2,000
Interest and dividends on investments
Taxable interest				17,654		10,643		$ 6,975
Interest exempt from U.S. federal income tax				334		348		279
Dividend income				312		223		134
Total interest and dividend income on investments				18,300		11,214		7,388
Gross realized investments losses, excluding losses from other-than-temporary impairment
Gross realized investment gains				324		323		860
Gross realized investment losses				(136)		(256)		(195)
Net realized gains on sales of investments				188		67		665
Securities available-for-sale
Allowance for credit losses				8		3
Fair value				256,936		249,679
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
Unallocated portfolio layer cumulative basis adjustments				93
Gross unrealized gains excluding the effect of unallocated portfolio cumulative basis adjustments				368
Gross unrealized losses excluding the effect of unallocated portfolio cumulative basis adjustments				(683)
Less than 12 months				$ 861		3,997
Percentage of investments gross-unrealized loss position for less than a year, rated investment grade				82.00%
12 months or longer				$ 2,933		2,607
Percentage of gross-unrealized loss position for a year or more rated investment grade				96.00%
Fair value of securities transferred from AFS to HTM	[1],[2],[3]			$ 0		21,688		$ 0
Debt securities available-for-sale (AFS)
Investment Holdings [Line Items]
Investments				256,936		249,679
HTM debt securities
Investment Holdings [Line Items]
Investments				254,247		268,863
Non-marketable equity securities measured using the measurement alternative
Investment Holdings [Line Items]
Investments				1,639		1,676
U.S. government-sponsored agency guaranteed
Securities available-for-sale
Amortized cost				30,279		12,009
Gross unrealized gains				170		8
Gross unrealized losses				734		755
Allowance for credit losses				0		0
Fair value				29,715		11,262
Less than 12 months				86		412
12 months or longer				648		343
Residential
Securities available-for-sale
Amortized cost				426		488
Gross unrealized gains				0		0
Gross unrealized losses				3		3
Allowance for credit losses				0		0
Fair value				423		485
Less than 12 months				1		3
12 months or longer				2		0
Commercial
Securities available-for-sale
Amortized cost				1		2
Gross unrealized gains				0		0
Gross unrealized losses				0		0
Allowance for credit losses				0		0
Fair value				1		2
Less than 12 months				0		0
12 months or longer				0		0
Mortgage-backed securities
Securities available-for-sale
Amortized cost				30,706		12,499
Gross unrealized gains				170		8
Gross unrealized losses				737		758
Allowance for credit losses				0		0
Fair value				30,139		11,749
Less than 12 months				87		415
12 months or longer				650		343
U.S. Treasury
Securities available-for-sale
Amortized cost				81,684		94,732
Gross unrealized gains				59		50
Gross unrealized losses				1,382		2,492
Allowance for credit losses				0		0
Fair value				80,361		92,290
Less than 12 months				113		1,001
12 months or longer				1,269		1,491
Agency obligations
Securities available-for-sale
Amortized cost				0		0
Gross unrealized gains				0		0
Gross unrealized losses				0		0
Allowance for credit losses				0		0
Fair value				0		0
U.S. Treasury and federal agency securities
Securities available-for-sale
Amortized cost				81,684		94,732
Gross unrealized gains				59		50
Gross unrealized losses				1,382		2,492
Allowance for credit losses				0		0
Fair value				80,361		92,290
Less than 12 months				113		1,001
12 months or longer				1,269		1,491
State and municipal
Securities available-for-sale
Amortized cost				2,204		2,363
Gross unrealized gains				18		19
Gross unrealized losses				91		159
Allowance for credit losses				0		0
Fair value				2,131		2,223
Less than 12 months				17		31
12 months or longer				74		128
Foreign government
Securities available-for-sale
Amortized cost				132,045		135,648
Gross unrealized gains				528		569
Gross unrealized losses				1,375		2,940
Allowance for credit losses				0		0
Fair value				131,198		133,277
Less than 12 months				540		2,332
12 months or longer				835		608
Corporate
Securities available-for-sale
Amortized cost				5,610		5,146
Gross unrealized gains				18		19
Gross unrealized losses				208		246
Allowance for credit losses				8		3
Fair value				5,412		4,916
Less than 12 months				103		209
12 months or longer				105		37
Asset-backed securities
Securities available-for-sale
Amortized cost				921		1,022
Gross unrealized gains				17		12
Gross unrealized losses				0		4
Allowance for credit losses				0		0
Fair value				938		1,030
Less than 12 months				0		4
12 months or longer				0		0
Other debt securities
Securities available-for-sale
Amortized cost				6,754		4,198
Gross unrealized gains				4		1
Gross unrealized losses				1		5
Allowance for credit losses				0		0
Fair value				6,757		4,194
Less than 12 months				1		5
12 months or longer				0		0
Debt securities
Securities available-for-sale
Amortized cost				259,924		255,608
Gross unrealized gains				814		678
Gross unrealized losses				3,794		6,604
Allowance for credit losses				8		3
Fair value				256,936		249,679
Unallocated portfolio layer cumulative basis adjustments				200
Fair value | Marketable equity securities
Investment Holdings [Line Items]
Investments				258		429
Fair value | Non-marketable equity securities
Investment Holdings [Line Items]
Investments				508		466
Fair value | Non-marketable equity securities measured using the measurement alternative
Investment Holdings [Line Items]
Alternative investment funds, fair value				25		27
Carried at cost | Non-marketable equity securities
Investment Holdings [Line Items]
Investments				$ 5,497		$ 5,469

[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.