INVESTMENTS - Overview (Details) - USD ($) $ in Millions		1 Months Ended	12 Months Ended
	Jan. 01, 2023	Jan. 31, 2023	Dec. 31, 2024		Dec. 31, 2023		Dec. 31, 2022
Investment Holdings [Line Items]
Total investments			$ 476,657		$ 519,085
Accrued interest receivable			2,000
Interest and dividends on investments
Taxable interest			17,978		17,654		$ 10,643
Interest exempt from U.S. federal income tax			317		334		348
Dividend income			367		312		223
Total interest and dividend income on investments			18,662		18,300		11,214
Gross realized investments losses, excluding losses from other-than-temporary impairment
Gross realized investment gains			533		324		323
Gross realized investment losses			(205)		(136)		(256)
Net realized gains on sales of investments			328		188		67
Securities available-for-sale
Allowance for credit losses			6		8
Fair value			226,876		256,936
Transfer of investment securities from HTM to AFS	$ 3,300	$ 3,300	0	[1],[2],[3]	3,324	[1],[2],[3]	$ 0	[1],[2],[3]
Transfer of securities from HTM to AFS, unrealized gain		$ 100
Unallocated portfolio layer cumulative basis adjustments			(72)
Gross unrealized gains excluding the effect of unallocated portfolio cumulative basis adjustments			35
Gross unrealized losses excluding the effect of unallocated portfolio cumulative basis adjustments			(1,129)
Less than 12 months			$ 1,037		861
Percentage of investments gross-unrealized loss position for less than a year, rated investment grade			67.00%
12 months or longer			$ 1,495		2,933
Percentage of gross-unrealized loss position for a year or more rated investment grade			91.00%
Debt securities available-for-sale (AFS)
Investment Holdings [Line Items]
Total investments			$ 226,876		256,936
HTM debt securities
Investment Holdings [Line Items]
Total investments			242,382		254,247
Non-marketable equity securities measured using the measurement alternative
Investment Holdings [Line Items]
Total investments			1,574		1,639
U.S. government-sponsored agency guaranteed
Securities available-for-sale
Amortized cost			30,208		30,279
Gross unrealized gains			40		170
Gross unrealized losses			942		734
Allowance for credit losses			0		0
Fair value			29,306		29,715
Less than 12 months			255		86
12 months or longer			687		648
Residential
Securities available-for-sale
Amortized cost			626		426
Gross unrealized gains			0		0
Gross unrealized losses			2		3
Allowance for credit losses			0		0
Fair value			624		423
Less than 12 months			1		1
12 months or longer			1		2
Commercial
Securities available-for-sale
Amortized cost			1		1
Gross unrealized gains			0		0
Gross unrealized losses			0		0
Allowance for credit losses			0		0
Fair value			1		1
Less than 12 months			0		0
12 months or longer			0		0
Mortgage-backed securities
Securities available-for-sale
Amortized cost			30,835		30,706
Gross unrealized gains			40		170
Gross unrealized losses			944		737
Allowance for credit losses			0		0
Fair value			29,931		30,139
Less than 12 months			256		87
12 months or longer			688		650
U.S. Treasury
Securities available-for-sale
Amortized cost			52,630		81,684
Gross unrealized gains			13		59
Gross unrealized losses			264		1,382
Allowance for credit losses			0		0
Fair value			52,379		80,361
Less than 12 months			166		113
12 months or longer			98		1,269
U.S. Treasury and federal agency securities
Securities available-for-sale
Amortized cost			52,630		81,684
Gross unrealized gains			13		59
Gross unrealized losses			264		1,382
Allowance for credit losses			0		0
Fair value			52,379		80,361
Less than 12 months			166		113
12 months or longer			98		1,269
State and municipal
Securities available-for-sale
Amortized cost			1,749		2,204
Gross unrealized gains			12		18
Gross unrealized losses			103		91
Allowance for credit losses			0		0
Fair value			1,658		2,131
Less than 12 months			72		17
12 months or longer			31		74
Foreign government
Securities available-for-sale
Amortized cost			134,002		132,045
Gross unrealized gains			444		528
Gross unrealized losses			1,087		1,375
Allowance for credit losses			0		0
Fair value			133,359		131,198
Less than 12 months			487		540
12 months or longer			600		835
Corporate
Securities available-for-sale
Amortized cost			4,923		5,610
Gross unrealized gains			19		18
Gross unrealized losses			122		208
Allowance for credit losses			6		8
Fair value			4,814		5,412
Less than 12 months			45		103
12 months or longer			77		105
Asset-backed securities
Securities available-for-sale
Amortized cost			856		921
Gross unrealized gains			3		17
Gross unrealized losses			11		0
Allowance for credit losses			0		0
Fair value			848		938
Less than 12 months			11		0
12 months or longer			0		0
Other debt securities
Securities available-for-sale
Amortized cost			3,887		6,754
Gross unrealized gains			1		4
Gross unrealized losses			1		1
Allowance for credit losses			0		0
Fair value			3,887		6,757
Less than 12 months			0		1
12 months or longer			1		0
Debt securities
Securities available-for-sale
Amortized cost			228,882		259,924
Gross unrealized gains			532		814
Gross unrealized losses			2,532		3,794
Allowance for credit losses			6		8
Fair value			226,876		256,936
Unallocated portfolio layer cumulative basis adjustments			(200)
Fair value | Marketable equity securities
Investment Holdings [Line Items]
Total investments			151		258
Fair value | Non-marketable equity securities
Investment Holdings [Line Items]
Total investments			427		508
Fair value | Non-marketable equity securities measured using the measurement alternative
Investment Holdings [Line Items]
Alternative investment funds, fair value			23		25
Carried at cost | Non-marketable equity securities
Investment Holdings [Line Items]
Total investments			$ 5,247		$ 5,497

[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, 2024 and 2023.

[3] See Note 2.