CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Jun. 30, 2021	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Dec. 31, 2019
Assets
Cash and due from banks	$ 28,906		$ 26,349
Cash and due from banks—intercompany	0		0
Deposits with banks	294,902		283,266
Deposits with banks—intercompany	0		0
Securities borrowed or purchased under agreements to resell	337,696		294,712
Securities borrowed and purchased under resale agreements—intercompany	0		0
Trading account assets	342,914		375,079
Trading account assets—intercompany	0		0
Investments	500,849		447,359
Loans, net of unearned income	664,764		675,883
Loans, net of unearned income—intercompany	0		0
Allowance for credit losses on loans (ACLL)	(17,715)	$ (19,238)	(24,956)	$ (26,426)	$ (26,298)	$ (12,783)
Total loans, net	647,049		650,927
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	209,560		182,398
Other assets—intercompany	0		0
Total assets	2,361,876		2,260,090
Liabilities and equity
Deposits	1,347,528		1,280,671
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $75,262 and $60,206 as of September 30, 2021 and December 31, 2020, respectively, at fair value)	209,184		199,525
Securities loaned and sold under repurchase agreements—intercompany	0		0
Trading account liabilities	179,286		168,027
Trading account liabilities—intercompany	0		0
Short-term borrowings	29,683		29,514
Short-term borrowings—intercompany	0		0
Long-term debt	258,274		271,686
Long-term debt—intercompany	0		0
Advances from subsidiaries	0		0
Other liabilities, including allowance	136,311		110,467
Other liabilities—intercompany	0		0
Stockholders’ equity	201,610		200,200	$ 194,595
Total liabilities and equity	2,361,876		2,260,090
Increase in other assets	123,947		110,683
Citigroup parent company
Liabilities and equity
Long-term debt	170,104		170,563
Citibank, N.A.
Liabilities and equity
Increase in other assets	35,000		29,500
Citibank, N.A. | Up to 30 days
Liabilities and equity
Placements with term of less than 30 days	19,500		24,300
Reportable legal entities | Citigroup parent company
Assets
Cash and due from banks	0		0
Cash and due from banks—intercompany	8		16
Deposits with banks	0		0
Deposits with banks—intercompany	4,000		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	249		307
Trading account assets—intercompany	1,001		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	147,338		152,383
Investments in subsidiaries	222,021		213,267
Other assets	11,043		12,156
Other assets—intercompany	2,867		2,781
Total assets	388,528		386,134
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $75,262 and $60,206 as of September 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	32		0
Trading account liabilities—intercompany	632		397
Short-term borrowings	0		0
Short-term borrowings—intercompany	0		0
Long-term debt	170,104		170,563
Long-term debt—intercompany	0		0
Advances from subsidiaries	14,049		12,975
Other liabilities, including allowance	2,780		2,692
Other liabilities—intercompany	56		65
Stockholders’ equity	200,875		199,442
Total liabilities and equity	388,528		386,134
Reportable legal entities | CGMHI
Assets
Cash and due from banks	965		628
Cash and due from banks—intercompany	6,033		6,081
Deposits with banks	5,763		5,224
Deposits with banks—intercompany	9,200		8,179
Securities borrowed or purchased under agreements to resell	277,955		238,718
Securities borrowed and purchased under resale agreements—intercompany	25,137		24,309
Trading account assets	204,591		222,278
Trading account assets—intercompany	5,043		9,400
Investments	266		374
Loans, net of unearned income	3,018		2,524
Loans, net of unearned income—intercompany	0		0
Allowance for credit losses on loans (ACLL)	0		0
Total loans, net	3,018		2,524
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	74,168		60,273
Other assets—intercompany	62,048		51,489
Total assets	674,187		629,477
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $75,262 and $60,206 as of September 30, 2021 and December 31, 2020, respectively, at fair value)	192,276		184,786
Securities loaned and sold under repurchase agreements—intercompany	66,703		76,590
Trading account liabilities	131,565		113,100
Trading account liabilities—intercompany	4,565		8,591
Short-term borrowings	14,547		12,323
Short-term borrowings—intercompany	16,400		12,757
Long-term debt	57,826		47,732
Long-term debt—intercompany	74,016		67,322
Advances from subsidiaries	0		0
Other liabilities, including allowance	66,345		55,217
Other liabilities—intercompany	12,466		15,378
Stockholders’ equity	37,478		35,681
Total liabilities and equity	674,187		629,477
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Assets
Cash and due from banks	27,941		25,721
Cash and due from banks—intercompany	(6,041)		(6,097)
Deposits with banks	289,139		278,042
Deposits with banks—intercompany	(13,200)		(12,679)
Securities borrowed or purchased under agreements to resell	59,741		55,994
Securities borrowed and purchased under resale agreements—intercompany	(25,137)		(24,309)
Trading account assets	138,074		152,494
Trading account assets—intercompany	(6,044)		(10,123)
Investments	500,582		446,984
Loans, net of unearned income	661,746		673,359
Loans, net of unearned income—intercompany	0		0
Allowance for credit losses on loans (ACLL)	(17,715)		(24,956)
Total loans, net	644,031		648,403
Advances to subsidiaries	(147,338)		(152,383)
Investments in subsidiaries	0		0
Other assets	124,349		109,969
Other assets—intercompany	(64,915)		(54,270)
Total assets	1,521,182		1,457,746
Liabilities and equity
Deposits	1,347,528		1,280,671
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $75,262 and $60,206 as of September 30, 2021 and December 31, 2020, respectively, at fair value)	16,908		14,739
Securities loaned and sold under repurchase agreements—intercompany	(66,703)		(76,590)
Trading account liabilities	47,689		54,927
Trading account liabilities—intercompany	(5,197)		(8,988)
Short-term borrowings	15,136		17,191
Short-term borrowings—intercompany	(16,400)		(12,757)
Long-term debt	30,344		53,391
Long-term debt—intercompany	(74,016)		(67,322)
Advances from subsidiaries	(14,049)		(12,975)
Other liabilities, including allowance	67,186		52,558
Other liabilities—intercompany	(12,522)		(15,443)
Stockholders’ equity	185,278		178,344
Total liabilities and equity	1,521,182		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	(222,021)		(213,267)
Other assets	0		0
Other assets—intercompany	0		0
Total assets	(222,021)		(213,267)
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $75,262 and $60,206 as of September 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	(222,021)		(213,267)
Total liabilities and equity	$ (222,021)		$ (213,267)