CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets
Cash and due from banks	$ 25,727		$ 23,775
Cash and due from banks—intercompany	0		0
Deposits with banks	173,559		156,741
Deposits with banks—intercompany	0		0
Federal funds sold and securities borrowed and purchased under agreements to resell (including $178,442 and $132,949 as of September 30, 2018 and December 31, 2017, respectively, at fair value)	280,941		232,478
Federal funds sold and resale agreements—intercompany	0		0
Trading account assets	257,502		252,790
Trading account assets—intercompany	0		0
Investments	345,513		352,290
Loans, net of unearned income	674,909		667,034
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	(12,336)	$ (12,126)	(12,355)	$ (12,366)	$ (12,025)	$ (12,060)
Total loans, net	662,573		654,679
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	179,350		169,712
Other assets—intercompany	0		0
Total assets	1,925,165		1,842,465
Liabilities and equity
Deposits	1,005,176		959,822
Deposits—intercompany	0		0
Federal funds purchased and securities loaned and sold under agreements to repurchase (including $48,148 and $40,638 as of September 30, 2018 and December 31, 2017, respectively, at fair value)	175,915		156,277
Federal funds purchased and securities loaned and sold—intercompany	0		0
Trading account liabilities	147,652		125,170
Trading account liabilities—intercompany	0		0
Short-term borrowings	33,770		44,452
Short-term borrowings—intercompany	0		0
Long-term debt	235,270		236,709
Long-term debt—intercompany	0		0
Advances from subsidiaries	0		0
Other liabilities	129,519		118,363
Other liabilities—intercompany	0		0
Stockholders’ equity	197,863		201,672	$ 228,622
Total liabilities and equity	1,925,165		1,842,465
Other	111,268		103,926
Citigroup parent company
Liabilities and equity
Long-term debt	148,183		152,163
Citibank, N.A.
Liabilities and equity
Other	30,900		29,700
Reportable legal entities | Citigroup parent company
Assets
Cash and due from banks	1		0
Cash and due from banks—intercompany	17		13
Deposits with banks	0		0
Deposits with banks—intercompany	3,000		11,000
Federal funds sold and securities borrowed and purchased under agreements to resell (including $178,442 and $132,949 as of September 30, 2018 and December 31, 2017, respectively, at fair value)	0		0
Federal funds sold and resale agreements—intercompany	0		0
Trading account assets	258		0
Trading account assets—intercompany	963		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	146,339		139,722
Investments in subsidiaries	203,896		210,537
Other assets	12,517		10,844
Other assets—intercompany	3,638		3,428
Total assets	370,636		375,609
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Federal funds purchased and securities loaned and sold under agreements to repurchase (including $48,148 and $40,638 as of September 30, 2018 and December 31, 2017, respectively, at fair value)	0		0
Federal funds purchased and securities loaned and sold—intercompany	0		0
Trading account liabilities	16		0
Trading account liabilities—intercompany	448		15
Short-term borrowings	254		251
Short-term borrowings—intercompany	0		0
Long-term debt	148,183		152,163
Long-term debt—intercompany	0		0
Advances from subsidiaries	21,965		19,136
Other liabilities	2,440		2,673
Other liabilities—intercompany	326		631
Stockholders’ equity	197,004		200,740
Total liabilities and equity	370,636		375,609
Reportable legal entities | CGMHI
Assets
Cash and due from banks	543		378
Cash and due from banks—intercompany	2,104		3,750
Deposits with banks	3,302		3,348
Deposits with banks—intercompany	6,386		5,219
Federal funds sold and securities borrowed and purchased under agreements to resell (including $178,442 and $132,949 as of September 30, 2018 and December 31, 2017, respectively, at fair value)	227,147		182,685
Federal funds sold and resale agreements—intercompany	19,572		16,091
Trading account assets	144,440		139,462
Trading account assets—intercompany	2,934		2,711
Investments	215		181
Loans, net of unearned income	1,518		900
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	0		0
Total loans, net	1,518		900
Advances to subsidiaries	0		0
Investments in subsidiaries	0		0
Other assets	67,087		58,299
Other assets—intercompany	45,654		43,613
Total assets	520,902		456,637
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Federal funds purchased and securities loaned and sold under agreements to repurchase (including $48,148 and $40,638 as of September 30, 2018 and December 31, 2017, respectively, at fair value)	154,341		134,888
Federal funds purchased and securities loaned and sold—intercompany	34,948		18,597
Trading account liabilities	94,163		80,801
Trading account liabilities—intercompany	3,143		2,182
Short-term borrowings	4,358		3,568
Short-term borrowings—intercompany	18,100		32,871
Long-term debt	24,324		18,048
Long-term debt—intercompany	65,811		60,765
Advances from subsidiaries	0		0
Other liabilities	73,178		62,113
Other liabilities—intercompany	16,369		9,753
Stockholders’ equity	32,167		33,051
Total liabilities and equity	520,902		456,637
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Assets
Cash and due from banks	25,183		23,397
Cash and due from banks—intercompany	(2,121)		(3,763)
Deposits with banks	170,257		153,393
Deposits with banks—intercompany	(9,386)		(16,219)
Federal funds sold and securities borrowed and purchased under agreements to resell (including $178,442 and $132,949 as of September 30, 2018 and December 31, 2017, respectively, at fair value)	53,794		49,793
Federal funds sold and resale agreements—intercompany	(19,572)		(16,091)
Trading account assets	112,804		113,328
Trading account assets—intercompany	(3,897)		(2,749)
Investments	345,291		352,082
Loans, net of unearned income	673,391		666,134
Loans, net of unearned income—intercompany	0		0
Allowance for loan losses	(12,336)		(12,355)
Total loans, net	661,055		653,779
Advances to subsidiaries	(146,339)		(139,722)
Investments in subsidiaries	0		0
Other assets	99,746		100,569
Other assets—intercompany	(49,292)		(47,041)
Total assets	1,237,523		1,220,756
Liabilities and equity
Deposits	1,005,176		959,822
Deposits—intercompany	0		0
Federal funds purchased and securities loaned and sold under agreements to repurchase (including $48,148 and $40,638 as of September 30, 2018 and December 31, 2017, respectively, at fair value)	21,574		21,389
Federal funds purchased and securities loaned and sold—intercompany	(34,948)		(18,597)
Trading account liabilities	53,473		44,369
Trading account liabilities—intercompany	(3,591)		(2,197)
Short-term borrowings	29,158		40,633
Short-term borrowings—intercompany	(18,100)		(32,871)
Long-term debt	62,763		66,498
Long-term debt—intercompany	(65,811)		(60,765)
Advances from subsidiaries	(21,965)		(19,136)
Other liabilities	53,901		53,577
Other liabilities—intercompany	(16,695)		(10,384)
Stockholders’ equity	172,588		178,418
Total liabilities and equity	1,237,523		1,220,756
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 securities borrowed and purchased under agreements to resell (including $178,442 and $132,949 as of September 30, 2018 and December 31, 2017, 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	(203,896)		(210,537)
Other assets	0		0
Other assets—intercompany	0		0
Total assets	(203,896)		(210,537)
Liabilities and equity
Deposits	0		0
Deposits—intercompany	0		0
Federal funds purchased and securities loaned and sold under agreements to repurchase (including $48,148 and $40,638 as of September 30, 2018 and December 31, 2017, respectively, 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	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	(203,896)		(210,537)
Total liabilities and equity	(203,896)		(210,537)
Up to 30 days | Citibank, N.A.
Liabilities and equity
Placements with term of less than 30 days	$ 18,100		$ 18,900