CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets
Cash and due from banks	$ 23,645	$ 23,775
Cash and due from banks—intercompany	0	0
Deposits with banks	164,460	156,741
Deposits with banks—intercompany	0	0
Federal funds sold and resale agreements	270,684	232,478
Federal funds sold and resale agreements—intercompany	0	0
Trading account assets	256,117	252,790
Trading account assets—intercompany	0	0
Investments	358,607	352,290
Loans, net of unearned income	684,196	667,034
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	(12,315)	(12,355)	$ (12,060)	$ (12,626)
Total loans, net	671,881	654,679
Advances to subsidiaries	0	0
Investments in subsidiaries	0	0
Other assets	171,989	169,712
Other assets—intercompany	0	0
Total assets	1,917,383	1,842,465
Liabilities and equity
Deposits	1,013,170	959,822
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase, at fair value	177,768	156,277
Federal funds purchased and securities loaned and sold—intercompany	0	0
Trading account liabilities	144,305	125,170
Trading account liabilities—intercompany	0	0
Short-term borrowings, at fair value	32,346	44,452
Short-term borrowings—intercompany	0	0
Long-term debt, at fair value	231,999	236,709
Long-term debt—intercompany	0	0
Advances from subsidiaries	0	0
Other liabilities	120,721	118,363
Other liabilities—intercompany	0	0
Stockholders’ equity	197,074	201,672	$ 226,143
Total liabilities and equity	1,917,383	1,842,465
Other assets	109,273	103,926
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 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	(205,337)	(210,537)
Other assets	0	0
Other assets—intercompany	0	0
Total assets	(205,337)	(210,537)
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 and 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	(205,337)	(210,537)
Total liabilities and equity	(205,337)	(210,537)
Citigroup parent company
Liabilities and equity
Long-term debt, at fair value	143,768
Other assets	34,700	29,700
Citigroup parent company | Reportable legal entities
Assets
Cash and due from banks	1	0
Cash and due from banks—intercompany	19	13
Deposits with banks	0	0
Deposits with banks—intercompany	3,000	11,000
Federal funds sold and resale agreements	0	0
Federal funds sold and resale agreements—intercompany	0	0
Trading account assets	302	0
Trading account assets—intercompany	627	38
Investments	7	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	143,119	139,722
Investments in subsidiaries	205,337	210,537
Other assets	9,861	10,844
Other assets—intercompany	3,037	3,428
Total assets	365,310	375,609
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 and sold—intercompany	0	0
Trading account liabilities	1	0
Trading account liabilities—intercompany	410	15
Short-term borrowings, at fair value	207	251
Short-term borrowings—intercompany	0	0
Long-term debt, at fair value	143,768	152,163
Long-term debt—intercompany	0	0
Advances from subsidiaries	21,471	19,136
Other liabilities	3,010	2,673
Other liabilities—intercompany	223	631
Stockholders’ equity	196,220	200,740
Total liabilities and equity	365,310	375,609
CGMHI | Reportable legal entities
Assets
Cash and due from banks	689	378
Cash and due from banks—intercompany	3,545	3,750
Deposits with banks	4,915	3,348
Deposits with banks—intercompany	6,528	5,219
Federal funds sold and resale agreements	212,720	182,685
Federal funds sold and resale agreements—intercompany	20,074	16,091
Trading account assets	146,233	139,462
Trading account assets—intercompany	1,728	2,711
Investments	224	181
Loans, net of unearned income	1,292	900
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	0	0
Total loans, net	1,292	900
Advances to subsidiaries	0	0
Investments in subsidiaries	0	0
Other assets	59,734	58,299
Other assets—intercompany	44,255	43,613
Total assets	501,937	456,637
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	155,830	134,888
Federal funds purchased and securities loaned and sold—intercompany	21,109	18,597
Trading account liabilities	95,571	80,801
Trading account liabilities—intercompany	1,398	2,182
Short-term borrowings, at fair value	3,656	3,568
Short-term borrowings—intercompany	11,343	32,871
Long-term debt, at fair value	25,986	18,048
Long-term debt—intercompany	73,884	60,765
Advances from subsidiaries	0	0
Other liabilities	66,732	62,113
Other liabilities—intercompany	13,763	9,753
Stockholders’ equity	32,665	33,051
Total liabilities and equity	501,937	456,637
Other Citigroup subsidiaries and eliminations | Reportable legal entities
Assets
Cash and due from banks	22,955	23,397
Cash and due from banks—intercompany	(3,564)	(3,763)
Deposits with banks	159,545	153,393
Deposits with banks—intercompany	(9,528)	(16,219)
Federal funds sold and resale agreements	57,964	49,793
Federal funds sold and resale agreements—intercompany	(20,074)	(16,091)
Trading account assets	109,582	113,328
Trading account assets—intercompany	(2,355)	(2,749)
Investments	358,376	352,082
Loans, net of unearned income	682,904	666,134
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	(12,315)	(12,355)
Total loans, net	670,589	653,779
Advances to subsidiaries	(143,119)	(139,722)
Investments in subsidiaries	0	0
Other assets	102,394	100,569
Other assets—intercompany	(47,292)	(47,041)
Total assets	1,255,473	1,220,756
Liabilities and equity
Deposits	1,013,170	959,822
Deposits—intercompany	0	0
Federal funds purchased and securities loaned or sold under agreements to repurchase, at fair value	21,938	21,389
Federal funds purchased and securities loaned and sold—intercompany	(21,109)	(18,597)
Trading account liabilities	48,733	44,369
Trading account liabilities—intercompany	(1,808)	(2,197)
Short-term borrowings, at fair value	28,483	40,633
Short-term borrowings—intercompany	(11,343)	(32,871)
Long-term debt, at fair value	62,245	66,498
Long-term debt—intercompany	(73,884)	(60,765)
Advances from subsidiaries	(21,471)	(19,136)
Other liabilities	50,979	53,577
Other liabilities—intercompany	(13,986)	(10,384)
Stockholders’ equity	173,526	178,418
Total liabilities and equity	1,255,473	1,220,756
Up to 30 days | Citigroup parent company
Liabilities and equity
Placements with term of less than 30 days	$ 22,400	$ 18,900