CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Assets
Cash and due from banks	$ 23,775	$ 23,043	$ 20,900	$ 32,108
Cash and due from banks—intercompany	0	0
Federal funds sold and resale agreements	232,478	236,813
Federal funds sold and resale agreements—intercompany	0	0
Trading account assets	251,556	243,925
Trading account assets—intercompany	0	0
Investments	352,290	353,304
Loans, net of unearned income	667,034	624,369
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	(12,355)	(12,060)	(12,626)	$ (15,994)
Total loans, net	654,679	612,309
Advances to subsidiaries	0	0
Investments in subsidiaries	0	0
Other assets	327,687	322,683
Other assets—intercompany	0	0
Total assets	1,842,465	1,792,077
Liabilities and equity
Deposits	959,822	929,406
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase, at fair value	156,277	141,821
Federal funds purchased and securities loaned or sold—intercompany	0	0
Trading account liabilities	124,047	139,045
Trading account liabilities—intercompany	0	0
Short-term borrowings, at fair value	44,452	30,701
Short-term borrowings—intercompany	0	0
Long-term debt, at fair value	236,709	206,178
Long-term debt—intercompany	0	0
Advances from subsidiaries	0	0
Other liabilities	119,486	118,783
Other liabilities—intercompany	0	0
Stockholders’ equity	201,672	226,143	$ 223,092
Total liabilities and equity	1,842,465	1,792,077
Other assets	105,160	128,008
Consolidating adjustments
Assets
Cash and due from banks	0	0
Cash and due from banks—intercompany	0	0
Federal funds sold and resale agreements	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	(210,537)	(226,279)
Other assets	0	0
Other assets—intercompany	0	0
Total assets	(210,537)	(226,279)
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase, 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, at fair value	0	0
Short-term borrowings—intercompany	0	0
Long-term debt, at fair value	0	0
Long-term debt—intercompany	0	0
Advances from subsidiaries	0	0
Other liabilities	0	0
Other liabilities—intercompany	0	0
Stockholders’ equity	(210,537)	(226,279)
Total liabilities and equity	(210,537)	(226,279)
Citigroup parent company
Liabilities and equity
Long-term debt, at fair value	152,163
Other assets	29,700	20,700
Citigroup parent company | Reportable legal entities
Assets
Cash and due from banks	0	0
Cash and due from banks—intercompany	13	142
Federal funds sold and resale agreements	0	0
Federal funds sold and resale agreements—intercompany	0	0
Trading account assets	0	6
Trading account assets—intercompany	38	1,173
Investments	27	173
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	139,722	143,154
Investments in subsidiaries	210,537	226,279
Other assets	10,844	23,734
Other assets—intercompany	14,428	27,845
Total assets	375,609	422,506
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase, 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	15	1,006
Short-term borrowings, at fair value	251	0
Short-term borrowings—intercompany	0	0
Long-term debt, at fair value	152,163	147,333
Long-term debt—intercompany	0	0
Advances from subsidiaries	19,136	41,258
Other liabilities	2,673	3,466
Other liabilities—intercompany	631	4,323
Stockholders’ equity	200,740	225,120
Total liabilities and equity	375,609	422,506
CGMHI | Reportable legal entities
Assets
Cash and due from banks	378	870
Cash and due from banks—intercompany	3,750	3,820
Federal funds sold and resale agreements	182,685	196,236
Federal funds sold and resale agreements—intercompany	16,091	12,270
Trading account assets	139,462	121,484
Trading account assets—intercompany	2,711	907
Investments	181	335
Loans, net of unearned income	900	575
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	0	0
Total loans, net	900	575
Advances to subsidiaries	0	0
Investments in subsidiaries	0	0
Other assets	61,647	46,095
Other assets—intercompany	48,832	38,207
Total assets	456,637	420,799
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase, at fair value	134,888	122,320
Federal funds purchased and securities loaned or sold—intercompany	18,597	25,417
Trading account liabilities	80,801	87,714
Trading account liabilities—intercompany	2,182	868
Short-term borrowings, at fair value	3,568	1,356
Short-term borrowings—intercompany	32,871	35,596
Long-term debt, at fair value	18,048	8,128
Long-term debt—intercompany	60,765	41,287
Advances from subsidiaries	0	0
Other liabilities	62,113	57,430
Other liabilities—intercompany	9,753	7,894
Stockholders’ equity	33,051	32,789
Total liabilities and equity	456,637	420,799
Other Citigroup subsidiaries and eliminations | Reportable legal entities
Assets
Cash and due from banks	23,397	22,173
Cash and due from banks—intercompany	(3,763)	(3,962)
Federal funds sold and resale agreements	49,793	40,577
Federal funds sold and resale agreements—intercompany	(16,091)	(12,270)
Trading account assets	112,094	122,435
Trading account assets—intercompany	(2,749)	(2,080)
Investments	352,082	352,796
Loans, net of unearned income	666,134	623,794
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	(12,355)	(12,060)
Total loans, net	653,779	611,734
Advances to subsidiaries	(139,722)	(143,154)
Investments in subsidiaries	0	0
Other assets	255,196	252,854
Other assets—intercompany	(63,260)	(66,052)
Total assets	1,220,756	1,175,051
Liabilities and equity
Deposits	959,822	929,406
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase, at fair value	21,389	19,501
Federal funds purchased and securities loaned or sold—intercompany	(18,597)	(25,417)
Trading account liabilities	43,246	51,331
Trading account liabilities—intercompany	(2,197)	(1,874)
Short-term borrowings, at fair value	40,633	29,345
Short-term borrowings—intercompany	(32,871)	(35,596)
Long-term debt, at fair value	66,498	50,717
Long-term debt—intercompany	(60,765)	(41,287)
Advances from subsidiaries	(19,136)	(41,258)
Other liabilities	54,700	57,887
Other liabilities—intercompany	(10,384)	(12,217)
Stockholders’ equity	178,418	194,513
Total liabilities and equity	1,220,756	1,175,051
Up to 30 days | Citigroup parent company
Liabilities and equity
Placements with term of less than 30 days	$ 18,900	$ 6,800