CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Cash and due from banks	$ 24,997		$ 23,645
Cash and due from banks—intercompany	0		0
Deposits with banks	178,246		164,460
Deposits with banks—intercompany	0		0
Securities borrowed and purchased under agreements to resell (including $178,108 and $147,701 as of June 30, 2019 and December 31, 2018, respectively, at fair value)	259,769		270,684
Securities borrowed and purchased under resale agreements—intercompany	0		0
Trading account assets	306,831		256,117
Trading account assets—intercompany	0		0
Investments	349,702		358,607
Loans, net of unearned income	688,670		684,196
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	(12,466)	$ (12,329)	(12,315)	$ (12,126)	$ (12,354)	$ (12,355)
Total loans, net	676,204		671,881
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	192,477		171,989
Other assets—intercompany	0		0
Total assets	1,988,226		1,917,383
Liabilities and equity
Deposits	1,045,607		1,013,170
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $45,137 and $44,510 as of June 30, 2019 and December 31, 2018, respectively, at fair value)	181,133		177,768
Securities loaned and sold under repurchase agreements—intercompany	0		0
Trading account liabilities	136,294		144,305
Trading account liabilities—intercompany	0		0
Short-term borrowings	42,442		32,346
Short-term borrowings—intercompany	0		0
Long-term debt	252,189		231,999
Long-term debt—intercompany	0		0
Advances from subsidiaries	0		0
Other liabilities	132,451		120,721
Other liabilities—intercompany	0		0
Stockholders’ equity	198,110		197,074	$ 200,968
Total liabilities and equity	1,988,226		1,917,383
Other	115,359		109,273
Citigroup parent company
Liabilities and equity
Long-term debt	152,141		143,767
Citibank, N.A.
Liabilities and equity
Other	51,900		34,700
Reportable legal entities | Citigroup parent company
Assets
Cash and due from banks	0		1
Cash and due from banks—intercompany	15		19
Deposits with banks	0		0
Deposits with banks—intercompany	3,000		3,000
Securities borrowed and purchased under agreements to resell (including $178,108 and $147,701 as of June 30, 2019 and December 31, 2018, respectively, at fair value)	0		0
Securities borrowed and purchased under resale agreements—intercompany	0		0
Trading account assets	323		302
Trading account assets—intercompany	1,682		627
Investments	1		7
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	146,408		143,119
Investments in subsidiaries	202,418		205,337
Other assets	11,700		9,861
Other assets—intercompany	3,726		3,037
Total assets	369,273		365,310
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $45,137 and $44,510 as of June 30, 2019 and December 31, 2018, respectively, at fair value)	0		0
Securities loaned and sold under repurchase agreements—intercompany	0		0
Trading account liabilities	6		1
Trading account liabilities—intercompany	3,159		410
Short-term borrowings	244		207
Short-term borrowings—intercompany	0		0
Long-term debt	152,141		143,768
Long-term debt—intercompany	0		0
Advances from subsidiaries	12,887		21,471
Other liabilities	3,243		3,010
Other liabilities—intercompany	234		223
Stockholders’ equity	197,359		196,220
Total liabilities and equity	369,273		365,310
Reportable legal entities | CGMHI
Assets
Cash and due from banks	689		689
Cash and due from banks—intercompany	3,790		3,545
Deposits with banks	4,420		4,915
Deposits with banks—intercompany	5,701		6,528
Securities borrowed and purchased under agreements to resell (including $178,108 and $147,701 as of June 30, 2019 and December 31, 2018, respectively, at fair value)	204,516		212,720
Securities borrowed and purchased under resale agreements—intercompany	18,767		20,074
Trading account assets	175,750		146,233
Trading account assets—intercompany	2,034		1,728
Investments	609		224
Loans, net of unearned income	1,955		1,292
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	0		0
Total loans, net	1,955		1,292
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	70,341		59,734
Other assets—intercompany	51,954		44,255
Total assets	540,526		501,937
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $45,137 and $44,510 as of June 30, 2019 and December 31, 2018, respectively, at fair value)	155,278		155,830
Securities loaned and sold under repurchase agreements—intercompany	42,564		21,109
Trading account liabilities	93,842		95,571
Trading account liabilities—intercompany	1,832		1,398
Short-term borrowings	8,633		3,656
Short-term borrowings—intercompany	20,190		11,343
Long-term debt	34,394		25,986
Long-term debt—intercompany	72,039		73,884
Advances from subsidiaries	0		0
Other liabilities	68,497		66,732
Other liabilities—intercompany	9,978		13,763
Stockholders’ equity	33,279		32,665
Total liabilities and equity	540,526		501,937
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Assets
Cash and due from banks	24,308		22,955
Cash and due from banks—intercompany	(3,805)		(3,564)
Deposits with banks	173,826		159,545
Deposits with banks—intercompany	(8,701)		(9,528)
Securities borrowed and purchased under agreements to resell (including $178,108 and $147,701 as of June 30, 2019 and December 31, 2018, respectively, at fair value)	55,253		57,964
Securities borrowed and purchased under resale agreements—intercompany	(18,767)		(20,074)
Trading account assets	130,758		109,582
Trading account assets—intercompany	(3,716)		(2,355)
Investments	349,092		358,376
Loans, net of unearned income	686,715		682,904
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	(12,466)		(12,315)
Total loans, net	674,249		670,589
Advances to subsidiaries	(146,408)		(143,119)
Investments in subsidiaries	0		0
Other assets	110,436		102,394
Other assets—intercompany	(55,680)		(47,292)
Total assets	1,280,845		1,255,473
Liabilities and equity
Deposits	1,045,607		1,013,170
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $45,137 and $44,510 as of June 30, 2019 and December 31, 2018, respectively, at fair value)	25,855		21,938
Securities loaned and sold under repurchase agreements—intercompany	(42,564)		(21,109)
Trading account liabilities	42,446		48,733
Trading account liabilities—intercompany	(4,991)		(1,808)
Short-term borrowings	33,565		28,483
Short-term borrowings—intercompany	(20,190)		(11,343)
Long-term debt	65,654		62,245
Long-term debt—intercompany	(72,039)		(73,884)
Advances from subsidiaries	(12,887)		(21,471)
Other liabilities	60,711		50,979
Other liabilities—intercompany	(10,212)		(13,986)
Stockholders’ equity	169,890		173,526
Total liabilities and equity	1,280,845		1,255,473
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 and purchased under agreements to resell (including $178,108 and $147,701 as of June 30, 2019 and December 31, 2018, respectively, at fair value)	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 loan losses	0		0
Total loans, net	0		0
Advances to subsidiaries	0		0
Investments in subsidiaries	(202,418)		(205,337)
Other assets	0		0
Other assets—intercompany	0		0
Total assets	(202,418)		(205,337)
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Securities loaned and sold under agreements to repurchase (including $45,137 and $44,510 as of June 30, 2019 and December 31, 2018, 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	0		0
Other liabilities—intercompany	0		0
Stockholders’ equity	(202,418)		(205,337)
Total liabilities and equity	(202,418)		(205,337)
Up to 30 days | Citibank, N.A.
Liabilities and equity
Placements with term of less than 30 days	$ 26,700		$ 22,400