CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period				$ 201,672
Balance, end of period		$ 200,968	$ 231,107	200,968	$ 231,107
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(671)	(45)	(2,051)	301
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		173	18	495	(108)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(2,219)	(75)	(1,158)	(799)
Adjustment to opening balance						$ (3)	$ 504
Adjusted balance, beginning of period						(1,161)	(295)
Total other comprehensive income		(498)	(27)	(1,556)	193
Balance, end of period		(2,717)	(102)	(2,717)	(102)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		418	(132)	585	(227)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(100)	48	(139)	83
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(793)	(412)	(921)	(352)
Adjustment to opening balance						0	0
Adjusted balance, beginning of period						(921)	(352)
Total other comprehensive income		318	(84)	446	(144)
Balance, end of period		(475)	(496)	(475)	(496)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		403	(219)	494	(221)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(102)	84	(105)	74
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(6,095)	(5,176)	(6,183)	(5,164)
Adjustment to opening balance						0	0
Adjusted balance, beginning of period						(6,183)	(5,164)
Total other comprehensive income		301	(135)	389	(147)
Balance, end of period		(5,794)	(5,311)	(5,794)	(5,311)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(2,869)	619	(1,739)	2,087
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		2	24	(8)	(126)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(24,588)	(24,188)	(25,708)	(25,506)
Adjustment to opening balance						0	0
Adjusted balance, beginning of period						(25,708)	(25,506)
Total other comprehensive income		(2,867)	643	(1,747)	1,961
Balance, end of period		(27,455)	(23,545)	(27,455)	(23,545)
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(4)	0	0	0
Adjustment to opening balance						0	0
Adjusted balance, beginning of period						0	0
Total other comprehensive income		(28)	0	(32)	0
Balance, end of period		(32)	0	(32)	0
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax		(41,519)	(39,514)	(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		(2,888)	408	(3,175)	2,126
Balance, end of period, pretax		(44,407)	(39,106)	(44,407)	(39,106)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		6,900	9,101	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		13	106	352	(148)
Balance, end of period, tax effect		6,913	9,207	6,913	9,207
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(34,619)	(30,413)	(34,668)	(32,381)
Adjustment to opening balance	[1]					(3)	504
Adjusted balance, beginning of period						$ (34,671)	$ (31,877)
Total other comprehensive income		(2,875)	514	(2,823)	1,978
Balance, end of period		(37,494)	(29,899)	(37,494)	(29,899)
Cash flow hedges | Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(132)	185	(422)	186
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		31	(68)	99	(71)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		(101)	117	(323)	115
Fair value hedges | Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(37)	0	(42)	0
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		9	0	10	0
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		$ (28)	$ 0	$ (32)	$ 0

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