CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015
Assets
Cash	$ 20,940		$ 23,043	$ 22,140		$ 20,900
Cash and due from banks—intercompany	0		0
Federal funds sold and securities borrowed or purchased under agreements to resell (including $142,831 and $133,204 as of June 30, 2017 and December 31, 2016, respectively, at fair value)	234,065		236,813
Federal funds sold and resale agreements—intercompany	0		0
Trading account assets	259,606		243,925
Trading account assets—intercompany	0		0
Investments	351,710		353,304
Loans, net of unearned income	644,695		624,369
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	(12,025)	$ (12,030)	(12,060)	(12,304)	$ (12,712)	$ (12,626)
Total loans, net	632,670		612,309
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	365,072		322,683
Other assets—intercompany	0		0
Total assets	1,864,063		1,792,077
Liabilities and equity
Deposits	958,743		929,406
Deposits—intercompany	0		0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $44,881 and $33,663 as of June 30, 2017 and December 31, 2016, respectively, at fair value)	154,780		141,821
Federal funds purchased and securities loaned or sold—intercompany	0		0
Trading account liabilities	136,745		139,045
Trading account liabilities—intercompany	0		0
Short-term borrowings	36,519		30,701
Short-term borrowings—intercompany	0		0
Long-term debt	225,179		206,178
Long-term debt—intercompany	0		0
Advances from subsidiaries	0		0
Other liabilities	120,990		118,783
Other liabilities—intercompany	0		0
Stockholders’ equity	231,107		226,143	$ 233,021
Total liabilities and equity	1,864,063		1,792,077
Other	131,647		128,008
Citigroup parent company
Liabilities and equity
Long-term debt	147,257		147,333
Citibank, N.A.
Liabilities and equity
Other	26,300		20,700
Reportable legal entities | Citigroup parent company
Assets
Cash	0		0
Cash and due from banks—intercompany	160		142
Federal funds sold and securities borrowed or purchased under agreements to resell (including $142,831 and $133,204 as of June 30, 2017 and December 31, 2016, respectively, at fair value)	0		0
Federal funds sold and resale agreements—intercompany	0		0
Trading account assets	11		6
Trading account assets—intercompany	1		1,173
Investments	26		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	132,366		143,154
Investments in subsidiaries	230,077		226,279
Other assets	23,712		23,734
Other assets—intercompany	15,650		27,845
Total assets	402,003		422,506
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $44,881 and $33,663 as of June 30, 2017 and December 31, 2016, 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	67		1,006
Short-term borrowings	201		0
Short-term borrowings—intercompany	0		0
Long-term debt	147,257		147,333
Long-term debt—intercompany	0		0
Advances from subsidiaries	20,761		41,258
Other liabilities	2,998		3,466
Other liabilities—intercompany	700		4,323
Stockholders’ equity	230,019		225,120
Total liabilities and equity	402,003		422,506
Reportable legal entities | CGMHI
Assets
Cash	793		870
Cash and due from banks—intercompany	2,843		3,820
Federal funds sold and securities borrowed or purchased under agreements to resell (including $142,831 and $133,204 as of June 30, 2017 and December 31, 2016, respectively, at fair value)	188,379		196,236
Federal funds sold and resale agreements—intercompany	15,478		12,270
Trading account assets	136,853		121,484
Trading account assets—intercompany	1,544		907
Investments	167		335
Loans, net of unearned income	891		575
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	0		0
Total loans, net	891		575
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	55,983		46,095
Other assets—intercompany	48,567		38,207
Total assets	451,498		420,799
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $44,881 and $33,663 as of June 30, 2017 and December 31, 2016, respectively, at fair value)	133,308		122,320
Federal funds purchased and securities loaned or sold—intercompany	18,993		25,417
Trading account liabilities	87,137		87,714
Trading account liabilities—intercompany	1,629		868
Short-term borrowings	3,217		1,356
Short-term borrowings—intercompany	57,532		35,596
Long-term debt	16,710		8,128
Long-term debt—intercompany	28,795		41,287
Advances from subsidiaries	0		0
Other liabilities	60,092		57,430
Other liabilities—intercompany	10,733		7,894
Stockholders’ equity	33,352		32,789
Total liabilities and equity	451,498		420,799
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Assets
Cash	20,147		22,173
Cash and due from banks—intercompany	(3,003)		(3,962)
Federal funds sold and securities borrowed or purchased under agreements to resell (including $142,831 and $133,204 as of June 30, 2017 and December 31, 2016, respectively, at fair value)	45,686		40,577
Federal funds sold and resale agreements—intercompany	(15,478)		(12,270)
Trading account assets	122,742		122,435
Trading account assets—intercompany	(1,545)		(2,080)
Investments	351,517		352,796
Loans, net of unearned income	643,804		623,794
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	(12,025)		(12,060)
Total loans, net	631,779		611,734
Advances to subsidiaries	(132,366)		(143,154)
Investments in subsidiaries	0		0
Other assets	285,377		252,854
Other assets—intercompany	(64,217)		(66,052)
Total assets	1,240,639		1,175,051
Liabilities and equity
Deposits	958,743		929,406
Deposits—intercompany	0		0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $44,881 and $33,663 as of June 30, 2017 and December 31, 2016, respectively, at fair value)	21,472		19,501
Federal funds purchased and securities loaned or sold—intercompany	(18,993)		(25,417)
Trading account liabilities	49,608		51,331
Trading account liabilities—intercompany	(1,696)		(1,874)
Short-term borrowings	33,101		29,345
Short-term borrowings—intercompany	(57,532)		(35,596)
Long-term debt	61,212		50,717
Long-term debt—intercompany	(28,795)		(41,287)
Advances from subsidiaries	(20,761)		(41,258)
Other liabilities	57,900		57,887
Other liabilities—intercompany	(11,433)		(12,217)
Stockholders’ equity	197,813		194,513
Total liabilities and equity	1,240,639		1,175,051
Consolidating adjustments
Assets
Cash	0		0
Cash and due from banks—intercompany	0		0
Federal funds sold and securities borrowed or purchased under agreements to resell (including $142,831 and $133,204 as of June 30, 2017 and December 31, 2016, 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	(230,077)		(226,279)
Other assets	0		0
Other assets—intercompany	0		0
Total assets	(230,077)		(226,279)
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $44,881 and $33,663 as of June 30, 2017 and December 31, 2016, 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	(230,077)		(226,279)
Total liabilities and equity	(230,077)		(226,279)
Up to 30 days | Citibank, N.A.
Liabilities and equity
Placements with term of less than 30 days	$ 23,400		$ 6,800