CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period				$ 201,672
Balance, end of period		$ 197,863	$ 228,622	197,863	$ 228,622
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax		(44,407)	(39,106)	(41,228)	(42,035)
Adjustment to opening balance, pretax							$ (4)		$ 803
Adjusted balance, beginning of period, pretax							(41,232)		(41,232)
Other comprehensive income (loss), pretax		(1,408)	(50)	(4,583)	2,076
Balance, end of period, pretax		(45,815)	(39,156)	(45,815)	(39,156)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		6,913	9,207	6,560	9,654
Adjustment to opening balance, tax effect							1		(299)
Adjusted balance, beginning of period, tax effect							6,561		9,355
Other comprehensive income (loss), tax effect		257	58	609	(90)
Balance, end of period, tax effect		7,170	9,265	7,170	9,265
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(37,494)	(29,899)	(34,668)	(32,381)
Adjustment to opening balance	[1]					$ 0	(3)	$ 0	504
Adjusted balance, beginning of period						$ (37,494)	(34,671)	$ (29,899)	(31,877)
Total other comprehensive income		(1,151)	8	(3,974)	1,986
Balance, end of period		(38,645)	(29,891)	(38,645)	(29,891)
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(810)	(107)	(2,861)	194
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		205	41	700	(67)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(2,717)	(102)	(1,158)	(799)
Adjustment to opening balance							(3)		504
Adjusted balance, beginning of period							(1,161)		(295)
Total other comprehensive income		(605)	(66)	(2,161)	127
Balance, end of period		(3,322)	(168)	(3,322)	(168)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(377)	(195)	208	(422)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		90	72	(49)	155
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(475)	(496)	(921)	(352)
Adjustment to opening balance							0		0
Adjusted balance, beginning of period							(921)		(352)
Total other comprehensive income		(287)	(123)	159	(267)
Balance, end of period		(762)	(619)	(762)	(619)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		55	(45)	549	(266)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(29)	16	(134)	90
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(5,794)	(5,311)	(6,183)	(5,164)
Adjustment to opening balance							0		0
Adjusted balance, beginning of period							(6,183)		(5,164)
Total other comprehensive income		26	(29)	415	(176)
Balance, end of period		(5,768)	(5,340)	(5,768)	(5,340)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(192)	285	(1,931)	2,372
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(29)	(67)	(37)	(193)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(27,455)	(23,545)	(25,708)	(25,506)
Adjustment to opening balance							0		0
Adjusted balance, beginning of period							(25,708)		(25,506)
Total other comprehensive income		(221)	218	(1,968)	2,179
Balance, end of period		(27,676)	(23,327)	(27,676)	(23,327)
Hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(32)	0	0	0
Adjustment to opening balance							0		0
Adjusted balance, beginning of period							$ 0		$ 0
Total other comprehensive income		10	0	(22)	0
Balance, end of period		(22)	0	(22)	0
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(97)	12	(519)	198
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		23	(4)	122	(75)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		(74)	8	(397)	123
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		13	0	(29)	0
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(3)	0	7	0
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		$ 10	$ 0	$ (22)	$ 0

[1] See Note 1 to the Consolidated Financial Statements for additional details.