CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2021	Mar. 31, 2021	Dec. 31, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019
Assets
Cash and due from banks	$ 27,117		$ 26,349
Cash and due from banks—intercompany	0		0
Deposits with banks	272,121		283,266
Deposits with banks—intercompany	0		0
Securities borrowed or purchased under agreements to resell	309,047		294,712
Securities borrowed and purchased under resale agreements—intercompany	0		0
Trading account assets	370,950		375,079
Trading account assets—intercompany	0		0
Investments	487,063		447,359
Loans, net of unearned income	676,834		675,883
Loans, net of unearned income—intercompany	0		0
Allowance for credit losses on loans (ACLL)	(19,238)	$ (21,638)	(24,956)	$ (26,298)	$ (20,380)	$ (12,783)
Total loans, net	657,596		650,927
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	203,974		182,398
Other assets—intercompany	0		0
Total assets	2,327,868		2,260,090
Liabilities and equity
Deposits	1,310,281		1,280,671
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $76,705 and $60,206 as of June 30, 2021 and December 31, 2020, respectively, at fair value)	221,817		199,525
Securities loaned and sold under repurchase agreements—intercompany	0		0
Trading account liabilities	174,706		168,027
Trading account liabilities—intercompany	0		0
Short-term borrowings	31,462		29,514
Short-term borrowings—intercompany	0		0
Long-term debt	264,575		271,686
Long-term debt—intercompany	0		0
Advances from subsidiaries	0		0
Other liabilities, including allowance	122,117		110,467
Other liabilities—intercompany	0		0
Stockholders’ equity	202,910		200,200	$ 192,386
Total liabilities and equity	2,327,868		2,260,090
Increase in other assets	116,089		110,683
Citigroup parent company
Liabilities and equity
Long-term debt	174,366		170,563
Citibank, N.A.
Liabilities and equity
Increase in other assets	42,000		29,500
Citibank, N.A. | Up to 30 days
Liabilities and equity
Placements with term of less than 30 days	31,800		24,300
Reportable legal entities | Citigroup parent company
Assets
Cash and due from banks	0		0
Cash and due from banks—intercompany	16		16
Deposits with banks	0		0
Deposits with banks—intercompany	3,000		4,500
Securities borrowed or purchased under agreements to resell	0		0
Securities borrowed and purchased under resale agreements—intercompany	0		0
Trading account assets	264		307
Trading account assets—intercompany	1,069		723
Investments	1		1
Loans, net of unearned income	0		0
Loans, net of unearned income—intercompany	0		0
Allowance for credit losses on loans (ACLL)	0		0
Total loans, net	0		0
Advances to subsidiaries	153,845		152,383
Investments in subsidiaries	220,810		213,267
Other assets	11,302		12,156
Other assets—intercompany	3,354		2,781
Total assets	393,661		386,134
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $76,705 and $60,206 as of June 30, 2021 and December 31, 2020, respectively, at fair value)	0		0
Securities loaned and sold under repurchase agreements—intercompany	0		0
Trading account liabilities	19		0
Trading account liabilities—intercompany	564		397
Short-term borrowings	0		0
Short-term borrowings—intercompany	0		0
Long-term debt	174,366		170,563
Long-term debt—intercompany	0		0
Advances from subsidiaries	13,747		12,975
Other liabilities, including allowance	2,806		2,692
Other liabilities—intercompany	0		65
Stockholders’ equity	202,159		199,442
Total liabilities and equity	393,661		386,134
Reportable legal entities | CGMHI
Assets
Cash and due from banks	723		628
Cash and due from banks—intercompany	5,919		6,081
Deposits with banks	7,398		5,224
Deposits with banks—intercompany	8,915		8,179
Securities borrowed or purchased under agreements to resell	251,864		238,718
Securities borrowed and purchased under resale agreements—intercompany	25,247		24,309
Trading account assets	222,808		222,278
Trading account assets—intercompany	9,759		9,400
Investments	265		374
Loans, net of unearned income	3,135		2,524
Loans, net of unearned income—intercompany	0		0
Allowance for credit losses on loans (ACLL)	0		0
Total loans, net	3,135		2,524
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	74,398		60,273
Other assets—intercompany	58,861		51,489
Total assets	669,292		629,477
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $76,705 and $60,206 as of June 30, 2021 and December 31, 2020, respectively, at fair value)	203,715		184,786
Securities loaned and sold under repurchase agreements—intercompany	48,508		76,590
Trading account liabilities	125,785		113,100
Trading account liabilities—intercompany	8,578		8,591
Short-term borrowings	15,681		12,323
Short-term borrowings—intercompany	18,337		12,757
Long-term debt	56,087		47,732
Long-term debt—intercompany	77,668		67,322
Advances from subsidiaries	0		0
Other liabilities, including allowance	61,526		55,217
Other liabilities—intercompany	16,004		15,378
Stockholders’ equity	37,403		35,681
Total liabilities and equity	669,292		629,477
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Assets
Cash and due from banks	26,394		25,721
Cash and due from banks—intercompany	(5,935)		(6,097)
Deposits with banks	264,723		278,042
Deposits with banks—intercompany	(11,915)		(12,679)
Securities borrowed or purchased under agreements to resell	57,183		55,994
Securities borrowed and purchased under resale agreements—intercompany	(25,247)		(24,309)
Trading account assets	147,878		152,494
Trading account assets—intercompany	(10,828)		(10,123)
Investments	486,797		446,984
Loans, net of unearned income	673,699		673,359
Loans, net of unearned income—intercompany	0		0
Allowance for credit losses on loans (ACLL)	(19,238)		(24,956)
Total loans, net	654,461		648,403
Advances to subsidiaries	(153,845)		(152,383)
Investments in subsidiaries	0		0
Other assets	118,274		109,969
Other assets—intercompany	(62,215)		(54,270)
Total assets	1,485,725		1,457,746
Liabilities and equity
Deposits	1,310,281		1,280,671
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $76,705 and $60,206 as of June 30, 2021 and December 31, 2020, respectively, at fair value)	18,102		14,739
Securities loaned and sold under repurchase agreements—intercompany	(48,508)		(76,590)
Trading account liabilities	48,902		54,927
Trading account liabilities—intercompany	(9,142)		(8,988)
Short-term borrowings	15,781		17,191
Short-term borrowings—intercompany	(18,337)		(12,757)
Long-term debt	34,122		53,391
Long-term debt—intercompany	(77,668)		(67,322)
Advances from subsidiaries	(13,747)		(12,975)
Other liabilities, including allowance	57,785		52,558
Other liabilities—intercompany	(16,004)		(15,443)
Stockholders’ equity	184,158		178,344
Total liabilities and equity	1,485,725		1,457,746
Consolidating adjustments
Assets
Cash and due from banks	0		0
Cash and due from banks—intercompany	0		0
Deposits with banks	0		0
Deposits with banks—intercompany	0		0
Securities borrowed or purchased under agreements to resell	0		0
Securities borrowed and purchased under resale agreements—intercompany	0		0
Trading account assets	0		0
Trading account assets—intercompany	0		0
Investments	0		0
Loans, net of unearned income	0		0
Loans, net of unearned income—intercompany	0		0
Allowance for credit losses on loans (ACLL)	0		0
Total loans, net	0		0
Advances to subsidiaries	0		0
Investments in subsidiaries	(220,810)		(213,267)
Other assets	0		0
Other assets—intercompany	0		0
Total assets	(220,810)		(213,267)
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $76,705 and $60,206 as of June 30, 2021 and December 31, 2020, respectively, at fair value)	0		0
Securities loaned and sold under repurchase agreements—intercompany	0		0
Trading account liabilities	0		0
Trading account liabilities—intercompany	0		0
Short-term borrowings	0		0
Short-term borrowings—intercompany	0		0
Long-term debt	0		0
Long-term debt—intercompany	0		0
Advances from subsidiaries	0		0
Other liabilities, including allowance	0		0
Other liabilities—intercompany	0		0
Stockholders’ equity	(220,810)		(213,267)
Total liabilities and equity	$ (220,810)		$ (213,267)