CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020	Dec. 31, 2019
Assets
Cash and due from banks	$ 26,204	$ 26,349
Cash and due from banks—intercompany	0	0
Deposits with banks	298,478	283,266
Deposits with banks—intercompany	0	0
Securities borrowed or purchased under agreements to resell	315,072	294,712
Securities borrowed and purchased under resale agreements—intercompany	0	0
Trading account assets	360,659	375,079
Trading account assets—intercompany	0	0
Investments	472,959	447,359
Loans, net of unearned income	665,988	675,883
Loans, net of unearned income—intercompany	0	0
Allowance for credit losses on loans (ACLL)	(21,638)	(24,956)	$ (20,380)	$ (12,783)
Total loans, net	644,350	650,927
Advances to subsidiaries	0	0
Investments in subsidiaries	0	0
Other assets	196,544	182,398
Other assets—intercompany	0	0
Total assets	2,314,266	2,260,090
Liabilities and equity
Deposits	1,300,975	1,280,671
Deposits—intercompany	0	0
Securities loaned and sold under agreements to repurchase (including $68,713 and $60,206 as of March 31, 2021 and December 31, 2020, respectively, at fair value)	219,168	199,525
Securities loaned and sold under repurchase agreements—intercompany	0	0
Trading account liabilities	179,117	168,027
Trading account liabilities—intercompany	0	0
Short-term borrowings	32,087	29,514
Short-term borrowings—intercompany	0	0
Long-term debt	256,335	271,686
Long-term debt—intercompany	0	0
Advances from subsidiaries	0	0
Other liabilities, including allowance	123,311	110,467
Other liabilities—intercompany	0	0
Stockholders’ equity	203,273	200,200	$ 193,326
Total liabilities and equity	2,314,266	2,260,090
Increase in other assets	109,433	110,683
Citigroup parent company
Liabilities and equity
Long-term debt	164,099	170,563
Citibank, N.A.
Liabilities and equity
Increase in other assets	31,600	29,500
Reportable legal entities | Citigroup parent company
Assets
Cash and due from banks	0	0
Cash and due from banks—intercompany	11	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	265	307
Trading account assets—intercompany	1,202	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	149,378	152,383
Investments in subsidiaries	218,488	213,267
Other assets	12,591	12,156
Other assets—intercompany	3,445	2,781
Total assets	388,381	386,134
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Securities loaned and sold under agreements to repurchase (including $68,713 and $60,206 as of March 31, 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	1,000	397
Short-term borrowings	0	0
Short-term borrowings—intercompany	0	0
Long-term debt	164,099	170,563
Long-term debt—intercompany	0	0
Advances from subsidiaries	17,937	12,975
Other liabilities, including allowance	2,695	2,692
Other liabilities—intercompany	69	65
Stockholders’ equity	202,549	199,442
Total liabilities and equity	388,381	386,134
Reportable legal entities | CGMHI
Assets
Cash and due from banks	676	628
Cash and due from banks—intercompany	5,929	6,081
Deposits with banks	5,408	5,224
Deposits with banks—intercompany	8,833	8,179
Securities borrowed or purchased under agreements to resell	258,976	238,718
Securities borrowed and purchased under resale agreements—intercompany	25,598	24,309
Trading account assets	222,114	222,278
Trading account assets—intercompany	11,732	9,400
Investments	235	374
Loans, net of unearned income	3,442	2,524
Loans, net of unearned income—intercompany	0	0
Allowance for credit losses on loans (ACLL)	0	0
Total loans, net	3,442	2,524
Advances to subsidiaries	0	0
Investments in subsidiaries	0	0
Other assets	72,333	60,273
Other assets—intercompany	54,272	51,489
Total assets	669,548	629,477
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Securities loaned and sold under agreements to repurchase (including $68,713 and $60,206 as of March 31, 2021 and December 31, 2020, respectively, at fair value)	201,562	184,786
Securities loaned and sold under repurchase agreements—intercompany	63,566	76,590
Trading account liabilities	129,449	113,100
Trading account liabilities—intercompany	11,181	8,591
Short-term borrowings	12,874	12,323
Short-term borrowings—intercompany	12,942	12,757
Long-term debt	50,267	47,732
Long-term debt—intercompany	72,433	67,322
Advances from subsidiaries	0	0
Other liabilities, including allowance	60,243	55,217
Other liabilities—intercompany	18,352	15,378
Stockholders’ equity	36,679	35,681
Total liabilities and equity	669,548	629,477
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Assets
Cash and due from banks	25,528	25,721
Cash and due from banks—intercompany	(5,940)	(6,097)
Deposits with banks	293,070	278,042
Deposits with banks—intercompany	(11,833)	(12,679)
Securities borrowed or purchased under agreements to resell	56,096	55,994
Securities borrowed and purchased under resale agreements—intercompany	(25,598)	(24,309)
Trading account assets	138,280	152,494
Trading account assets—intercompany	(12,934)	(10,123)
Investments	472,723	446,984
Loans, net of unearned income	662,546	673,359
Loans, net of unearned income—intercompany	0	0
Allowance for credit losses on loans (ACLL)	(21,638)	(24,956)
Total loans, net	640,908	648,403
Advances to subsidiaries	(149,378)	(152,383)
Investments in subsidiaries	0	0
Other assets	111,620	109,969
Other assets—intercompany	(57,717)	(54,270)
Total assets	1,474,825	1,457,746
Liabilities and equity
Deposits	1,300,975	1,280,671
Deposits—intercompany	0	0
Securities loaned and sold under agreements to repurchase (including $68,713 and $60,206 as of March 31, 2021 and December 31, 2020, respectively, at fair value)	17,606	14,739
Securities loaned and sold under repurchase agreements—intercompany	(63,566)	(76,590)
Trading account liabilities	49,636	54,927
Trading account liabilities—intercompany	(12,181)	(8,988)
Short-term borrowings	19,213	17,191
Short-term borrowings—intercompany	(12,942)	(12,757)
Long-term debt	41,969	53,391
Long-term debt—intercompany	(72,433)	(67,322)
Advances from subsidiaries	(17,937)	(12,975)
Other liabilities, including allowance	60,373	52,558
Other liabilities—intercompany	(18,421)	(15,443)
Stockholders’ equity	182,533	178,344
Total liabilities and equity	1,474,825	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	(218,488)	(213,267)
Other assets	0	0
Other assets—intercompany	0	0
Total assets	(218,488)	(213,267)
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Securities loaned and sold under agreements to repurchase (including $68,713 and $60,206 as of March 31, 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	(218,488)	(213,267)
Total liabilities and equity	(218,488)	(213,267)
Up to 30 days | Citibank, N.A.
Liabilities and equity
Placements with term of less than 30 days	$ 19,400	$ 24,300