CONSOLIDATED STATEMENT OF CHANGES IN STOCKHOLDERS' EQUITY - USD ($) shares in Thousands, $ in Millions		Total	Citigroup stockholders' equity	Preferred stock at aggregate liquidation value	Citigroup common stockholders' equity	Common stock and additional paid-in capital	Retained earnings		Treasury stock, at cost		Citigroup's accumulated other comprehensive income (loss)		Noncontrolling interests
Increase (Decrease) in Stockholders' Equity
Adjusted balance, beginning of year							$ 117,852				$ (23,216)
Balance, beginning of year at Dec. 31, 2014				$ 10,468		$ 108,010	117,852		$ (2,929)		(23,216)		$ 1,511
Balance, beginning of year (in shares) at Dec. 31, 2014				419		3,082,038			58,119
Increase (Decrease) in Stockholders' Equity
Issuance of new preferred stock				$ 6,250
Issuance of new preferred stock (in shares)				250
Employee benefit plans						$ 357			$ 704	[1]
Employee benefit plans (in shares)						17,438			13,318	[1]
Preferred stock issuance expense						$ (23)
Other						$ (25)							(41)
Other (in shares)						6
Net income attributable to noncontrolling-interest shareholders		$ 17,332					17,242						90
Common dividends	[2]						(484)
Preferred dividends		(769)					(769)
Transactions between Citigroup and the noncontrolling-interest shareholders													(164)
Treasury stock acquired	[3]								$ (5,452)
Treasure stock acquired (in shares)	[3]								(101,402)
Dividends paid to noncontrolling-interest shareholders													(78)
Other comprehensive income (loss)		(6,128)									(6,128)	[4]	(83)
Net change in noncontrolling interests													(276)
Balance, end of year at Dec. 31, 2015		223,092	$ 221,857	$ 16,718	$ 205,139	$ 108,319	133,841		$ (7,677)		(29,344)		1,235
Balance, end of year (in shares) at Dec. 31, 2015				669	2,953,279	3,099,482			146,203
Increase (Decrease) in Stockholders' Equity
Adjusted balance, beginning of year							133,856				(29,359)
Adjustment to opening balance, net of taxes at Dec. 31, 2015	[5]						15				(15)
Increase (Decrease) in Stockholders' Equity
Issuance of new preferred stock				$ 2,535
Issuance of new preferred stock (in shares)				101
Employee benefit plans						$ (251)			$ 826	[1]
Employee benefit plans (in shares)	[1]								14,256
Preferred stock issuance expense						(37)
Other						42							(36)
Net income attributable to noncontrolling-interest shareholders		14,975					14,912						63
Common dividends	[2]						(1,214)
Preferred dividends		(1,077)					(1,077)
Transactions between noncontrolling-interest shareholders and the related consolidated subsidiary													(11)
Transactions between Citigroup and the noncontrolling-interest shareholders													(130)
Treasury stock acquired	[3]								$ (9,451)
Treasure stock acquired (in shares)	[3]								(195,143)
Dividends paid to noncontrolling-interest shareholders													(42)
Other comprehensive income (loss)		(3,022)									(3,022)	[4]	(56)
Net change in noncontrolling interests													(212)
Balance, end of year at Dec. 31, 2016		226,143	225,120	$ 19,253	$ 205,867	$ 108,073	146,477		$ (16,302)		(32,381)		1,023
Balance, end of year (in shares) at Dec. 31, 2016				770	2,772,392	3,099,482			327,090
Increase (Decrease) in Stockholders' Equity
Adjusted balance, beginning of year							145,817				(31,877)
Adjustment to opening balance, net of taxes at Dec. 31, 2016	[5]						(660)				504
Increase (Decrease) in Stockholders' Equity
Employee benefit plans						$ (27)			$ 531	[1]
Employee benefit plans (in shares)						41			11,651	[1]
Other						$ (7)	(90)	[6]					(72)
Net income attributable to noncontrolling-interest shareholders		(6,738)					(6,798)						60
Common dividends	[2]						(2,595)
Preferred dividends		(1,213)					(1,213)
Impact of Tax Reform related to AOCI reclassification		(3,304)					3,304	[4]
Transactions between noncontrolling-interest shareholders and the related consolidated subsidiary													(28)
Transactions between Citigroup and the noncontrolling-interest shareholders													(121)
Treasury stock acquired	[3]								$ (14,538)
Treasure stock acquired (in shares)	[3]								(214,176)
Dividends paid to noncontrolling-interest shareholders													(44)
Other comprehensive income (loss)		(2,791)									(2,791)	[4]	114
Net change in noncontrolling interests													(91)
Balance, end of year at Dec. 31, 2017		$ 201,672	$ 200,740	$ 19,253	$ 181,487	$ 108,039	$ 138,425		$ (30,309)		$ (34,668)		$ 932
Balance, end of year (in shares) at Dec. 31, 2017				770	2,569,908	3,099,523			529,615

[1] Includes treasury stock related to (i) certain activity on employee stock option program exercises, where the employee delivers existing shares to cover the option exercise, or (ii) under Citi’s employee-restricted or deferred-stock programs, where shares are withheld to satisfy tax requirements.

[2] Common dividends declared were $0.16 per share in the first and second quarters and $0.32 per share in the third and fourth quarters of 2017; $0.05 per share in the first and second quarters and $0.16 per share in the third and fourth quarters of 2016; and $0.01 in the first quarter and $0.05 per share in the second, third and fourth quarters of 2015.

[3] For 2017, 2016 and 2015, primarily consists of open market purchases under Citi’s Board of Directors-approved common stock repurchase program.

[4] Includes the impact of ASU 2018-02, which transferred those amounts from AOCI to Retained earnings. See Notes 1 and 19 to the Consolidated Financial Statements.

[5] See Note 1 to the Consolidated Financial Statements.

[6] Includes the impact of ASU No. 2016-09. See Note 1 to the Consolidated Financial Statements.