CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets
Cash and due from banks	$ 21,850	$ 23,775
Cash and due from banks—intercompany	0	0
Deposits with banks	180,854	156,741
Deposits with banks - intercompany	0	0
Federal funds sold and securities borrowed or purchased under agreements to resell (including $161,537 and $132,949 as of March 31, 2018 and December 31, 2017, respectively, at fair value)	257,887	232,478
Federal funds sold and resale agreements—intercompany	0	0
Trading account assets	268,808	252,790
Trading account assets—intercompany	0	0
Investments	351,971	352,290
Loans, net of unearned income	672,938	667,034
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	(12,354)	(12,355)	$ (12,030)	$ (12,060)
Total loans, net	660,584	654,679
Advances to subsidiaries	0	0
Investments in subsidiaries	0	0
Other assets	180,150	169,712
Other assets—intercompany	0	0
Total assets	1,922,104	1,842,465
Liabilities and equity
Deposits	1,001,219	959,822
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $45,840 and $40,638 as of March 31, 2018 and December 31, 2017, respectively, at fair value)	171,759	156,277
Federal funds purchased and securities loaned or sold—intercompany	0	0
Trading account liabilities	143,961	125,170
Trading account liabilities—intercompany	0	0
Short-term borrowings	36,094	44,452
Short-term borrowings—intercompany	0	0
Long-term debt	237,938	236,709
Long-term debt—intercompany	0	0
Advances from subsidiaries	0	0
Other liabilities	128,267	118,363
Other liabilities—intercompany	0	0
Stockholders’ equity	202,866	201,672	$ 228,997
Total liabilities and equity	1,922,104	1,842,465
Other	105,882	103,926
Citigroup parent company
Liabilities and equity
Long-term debt	153,074	152,163
Citibank, N.A.
Liabilities and equity
Other	24,800	29,700
Reportable legal entities | Citigroup parent company
Assets
Cash and due from banks	2	0
Cash and due from banks—intercompany	14	13
Deposits with banks	0	0
Deposits with banks - intercompany	3,000	11,000
Federal funds sold and securities borrowed or purchased under agreements to resell (including $161,537 and $132,949 as of March 31, 2018 and December 31, 2017, respectively, at fair value)	0	0
Federal funds sold and resale agreements—intercompany	0	0
Trading account assets	298	0
Trading account assets—intercompany	483	38
Investments	7,755	27
Loans, net of unearned income	0	0
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	0	0
Total loans, net	0	0
Advances to subsidiaries	141,977	139,722
Investments in subsidiaries	209,808	210,537
Other assets	10,784	10,844
Other assets—intercompany	3,667	3,428
Total assets	377,788	375,609
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $45,840 and $40,638 as of March 31, 2018 and December 31, 2017, respectively, at fair value)	0	0
Federal funds purchased and securities loaned or sold—intercompany	0	0
Trading account liabilities	2	0
Trading account liabilities—intercompany	241	15
Short-term borrowings	235	251
Short-term borrowings—intercompany	0	0
Long-term debt	153,074	152,163
Long-term debt—intercompany	0	0
Advances from subsidiaries	19,151	19,136
Other liabilities	2,832	2,673
Other liabilities—intercompany	338	631
Stockholders’ equity	201,915	200,740
Total liabilities and equity	377,788	375,609
Reportable legal entities | CGMHI
Assets
Cash and due from banks	423	378
Cash and due from banks—intercompany	5,225	3,750
Deposits with banks	3,005	3,348
Deposits with banks - intercompany	5,492	5,219
Federal funds sold and securities borrowed or purchased under agreements to resell (including $161,537 and $132,949 as of March 31, 2018 and December 31, 2017, respectively, at fair value)	206,659	182,685
Federal funds sold and resale agreements—intercompany	14,284	16,091
Trading account assets	150,249	139,462
Trading account assets—intercompany	2,079	2,711
Investments	249	181
Loans, net of unearned income	875	900
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	0	0
Total loans, net	875	900
Advances to subsidiaries	0	0
Investments in subsidiaries	0	0
Other assets	66,723	58,299
Other assets—intercompany	47,051	43,613
Total assets	502,314	456,637
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $45,840 and $40,638 as of March 31, 2018 and December 31, 2017, respectively, at fair value)	144,400	134,888
Federal funds purchased and securities loaned or sold—intercompany	20,444	18,597
Trading account liabilities	98,287	80,801
Trading account liabilities—intercompany	1,904	2,182
Short-term borrowings	3,159	3,568
Short-term borrowings—intercompany	41,097	32,871
Long-term debt	19,907	18,048
Long-term debt—intercompany	60,351	60,765
Advances from subsidiaries	0	0
Other liabilities	69,516	62,113
Other liabilities—intercompany	10,336	9,753
Stockholders’ equity	32,913	33,051
Total liabilities and equity	502,314	456,637
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Assets
Cash and due from banks	21,425	23,397
Cash and due from banks—intercompany	(5,239)	(3,763)
Deposits with banks	177,849	153,393
Deposits with banks - intercompany	(8,492)	(16,219)
Federal funds sold and securities borrowed or purchased under agreements to resell (including $161,537 and $132,949 as of March 31, 2018 and December 31, 2017, respectively, at fair value)	51,228	49,793
Federal funds sold and resale agreements—intercompany	(14,284)	(16,091)
Trading account assets	118,261	113,328
Trading account assets—intercompany	(2,562)	(2,749)
Investments	343,967	352,082
Loans, net of unearned income	672,063	666,134
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	(12,354)	(12,355)
Total loans, net	659,709	653,779
Advances to subsidiaries	(141,977)	(139,722)
Investments in subsidiaries	0	0
Other assets	102,643	100,569
Other assets—intercompany	(50,718)	(47,041)
Total assets	1,251,810	1,220,756
Liabilities and equity
Deposits	1,001,219	959,822
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $45,840 and $40,638 as of March 31, 2018 and December 31, 2017, respectively, at fair value)	27,359	21,389
Federal funds purchased and securities loaned or sold—intercompany	(20,444)	(18,597)
Trading account liabilities	45,672	44,369
Trading account liabilities—intercompany	(2,145)	(2,197)
Short-term borrowings	32,700	40,633
Short-term borrowings—intercompany	(41,097)	(32,871)
Long-term debt	64,957	66,498
Long-term debt—intercompany	(60,351)	(60,765)
Advances from subsidiaries	(19,151)	(19,136)
Other liabilities	55,919	53,577
Other liabilities—intercompany	(10,674)	(10,384)
Stockholders’ equity	177,846	178,418
Total liabilities and equity	1,251,810	1,220,756
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
Federal funds sold and securities borrowed or purchased under agreements to resell (including $161,537 and $132,949 as of March 31, 2018 and December 31, 2017, respectively, at fair value)	0	0
Federal funds sold and 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 loan losses	0	0
Total loans, net	0	0
Advances to subsidiaries	0	0
Investments in subsidiaries	(209,808)	(210,537)
Other assets	0	0
Other assets—intercompany	0	0
Total assets	(209,808)	(210,537)
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $45,840 and $40,638 as of March 31, 2018 and December 31, 2017, respectively, at fair value)	0	0
Federal funds purchased and securities loaned or sold—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	0	0
Other liabilities—intercompany	0	0
Stockholders’ equity	(209,808)	(210,537)
Total liabilities and equity	(209,808)	(210,537)
Up to 30 days | Citibank, N.A.
Liabilities and equity
Placements with term of less than 30 days	$ 20,300	$ 18,900