CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets
Cash	$ 22,604		$ 23,043	$ 23,419		$ 20,900
Cash and due from banks—intercompany	0		0
Federal funds sold and securities borrowed or purchased under agreements to resell (including $156,332 and $133,204 as of September 30, 2017 and December 31, 2016, respectively, at fair value)	252,608		236,813
Federal funds sold and resale agreements—intercompany	0		0
Trading account assets	258,907		243,925
Trading account assets—intercompany	0		0
Investments	354,674		353,304
Loans, net of unearned income	653,183		624,369
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	(12,366)	$ (12,025)	(12,060)	(12,439)	$ (12,304)	$ (12,626)
Total loans, net	640,817		612,309
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	359,523		322,683
Other assets—intercompany	0		0
Total assets	1,889,133		1,792,077
Liabilities and equity
Deposits	964,038		929,406
Deposits—intercompany	0		0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $45,325 and $33,663 as of September 30, 2017 and December 31, 2016, respectively, at fair value)	161,282		141,821
Federal funds purchased and securities loaned or sold—intercompany	0		0
Trading account liabilities	138,820		139,045
Trading account liabilities—intercompany	0		0
Short-term borrowings	38,149		30,701
Short-term borrowings—intercompany	0		0
Long-term debt	232,673		206,178
Long-term debt—intercompany	0		0
Advances from subsidiaries	0		0
Other liabilities	125,549		118,783
Other liabilities—intercompany	0		0
Stockholders’ equity	228,622		226,143	$ 232,690
Total liabilities and equity	1,889,133		1,792,077
Other	130,312		128,008
Citigroup parent company
Liabilities and equity
Long-term debt	151,914		147,333
Citibank, N.A.
Liabilities and equity
Other	17,800		20,700
Reportable legal entities | Citigroup parent company
Assets
Cash	0		0
Cash and due from banks—intercompany	179		142
Federal funds sold and securities borrowed or purchased under agreements to resell (including $156,332 and $133,204 as of September 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		6
Trading account assets—intercompany	215		1,173
Investments	28		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	132,197		143,154
Investments in subsidiaries	229,142		226,279
Other assets	24,032		23,734
Other assets—intercompany	15,541		27,845
Total assets	401,334		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 $45,325 and $33,663 as of September 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	18		1,006
Short-term borrowings	246		0
Short-term borrowings—intercompany	0		0
Long-term debt	151,914		147,333
Long-term debt—intercompany	0		0
Advances from subsidiaries	17,947		41,258
Other liabilities	2,790		3,466
Other liabilities—intercompany	785		4,323
Stockholders’ equity	227,634		225,120
Total liabilities and equity	401,334		422,506
Reportable legal entities | CGMHI
Assets
Cash	728		870
Cash and due from banks—intercompany	3,791		3,820
Federal funds sold and securities borrowed or purchased under agreements to resell (including $156,332 and $133,204 as of September 30, 2017 and December 31, 2016, respectively, at fair value)	202,366		196,236
Federal funds sold and resale agreements—intercompany	14,980		12,270
Trading account assets	137,196		121,484
Trading account assets—intercompany	1,208		907
Investments	162		335
Loans, net of unearned income	1,364		575
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	0		0
Total loans, net	1,364		575
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	58,665		46,095
Other assets—intercompany	49,032		38,207
Total assets	469,492		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 $45,325 and $33,663 as of September 30, 2017 and December 31, 2016, respectively, at fair value)	135,520		122,320
Federal funds purchased and securities loaned or sold—intercompany	19,127		25,417
Trading account liabilities	91,058		87,714
Trading account liabilities—intercompany	1,071		868
Short-term borrowings	3,221		1,356
Short-term borrowings—intercompany	63,197		35,596
Long-term debt	17,758		8,128
Long-term debt—intercompany	30,609		41,287
Advances from subsidiaries	0		0
Other liabilities	62,950		57,430
Other liabilities—intercompany	11,281		7,894
Stockholders’ equity	33,700		32,789
Total liabilities and equity	469,492		420,799
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Assets
Cash	21,876		22,173
Cash and due from banks—intercompany	(3,970)		(3,962)
Federal funds sold and securities borrowed or purchased under agreements to resell (including $156,332 and $133,204 as of September 30, 2017 and December 31, 2016, respectively, at fair value)	50,242		40,577
Federal funds sold and resale agreements—intercompany	(14,980)		(12,270)
Trading account assets	121,711		122,435
Trading account assets—intercompany	(1,423)		(2,080)
Investments	354,484		352,796
Loans, net of unearned income	651,819		623,794
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	(12,366)		(12,060)
Total loans, net	639,453		611,734
Advances to subsidiaries	(132,197)		(143,154)
Investments in subsidiaries	0		0
Other assets	276,826		252,854
Other assets—intercompany	(64,573)		(66,052)
Total assets	1,247,449		1,175,051
Liabilities and equity
Deposits	964,038		929,406
Deposits—intercompany	0		0
Federal funds purchased and securities loaned or sold under agreements to repurchase (including $45,325 and $33,663 as of September 30, 2017 and December 31, 2016, respectively, at fair value)	25,762		19,501
Federal funds purchased and securities loaned or sold—intercompany	(19,127)		(25,417)
Trading account liabilities	47,762		51,331
Trading account liabilities—intercompany	(1,089)		(1,874)
Short-term borrowings	34,682		29,345
Short-term borrowings—intercompany	(63,197)		(35,596)
Long-term debt	63,001		50,717
Long-term debt—intercompany	(30,609)		(41,287)
Advances from subsidiaries	(17,947)		(41,258)
Other liabilities	59,809		57,887
Other liabilities—intercompany	(12,066)		(12,217)
Stockholders’ equity	196,430		194,513
Total liabilities and equity	1,247,449		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 $156,332 and $133,204 as of September 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	(229,142)		(226,279)
Other assets	0		0
Other assets—intercompany	0		0
Total assets	(229,142)		(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 $45,325 and $33,663 as of September 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	(229,142)		(226,279)
Total liabilities and equity	(229,142)		(226,279)
Up to 30 days | Citibank, N.A.
Liabilities and equity
Placements with term of less than 30 days	$ 16,000		$ 6,800