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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Jan. 01, 2016	Dec. 31, 2015
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period				$ 226,143
Balance, end of period		$ 231,107	$ 233,021	231,107	$ 233,021
Reclassification from AOCI		29,899		29,899		$ 32,381
Retained earnings		152,178		152,178		146,477
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(45)	1,482	301	4,706
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		18	(555)	(108)	(1,745)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(75)	1,127	(799)	(907)
Adjustment to opening balance						504		$ 0
Adjusted balance, beginning of period						(295)		(907)
Total other comprehensive income		(27)	927	193	2,961
Balance, end of period		(102)	2,054	(102)	2,054
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(132)	20	(227)	327
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		48	(8)	83	(122)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(412)	178	(352)	0
Adjustment to opening balance						0		(15)
Adjusted balance, beginning of period						(352)		(15)
Total other comprehensive income		(84)	12	(144)	205
Balance, end of period		(496)	190	(496)	190
Cash flow hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		185	257	186	739
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(68)	(106)	(71)	(271)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(562)	(300)	(560)	(617)
Adjustment to opening balance						0		0
Adjusted balance, beginning of period						(560)		(617)
Total other comprehensive income		117	151	115	468
Balance, end of period		(445)	(149)	(445)	(149)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(219)	(31)	(221)	(758)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		84	4	74	266
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(5,176)	(5,581)	(5,164)	(5,116)
Adjustment to opening balance						0		0
Adjusted balance, beginning of period						(5,164)		(5,116)
Total other comprehensive income		(135)	(27)	(147)	(492)
Balance, end of period		(5,311)	(5,608)	(5,311)	(5,608)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		619	(774)	2,087	(262)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		24	222	(126)	364
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(24,188)	(22,050)	(25,506)	(22,704)
Adjustment to opening balance						0		0
Adjusted balance, beginning of period						(25,506)		(22,704)
Total other comprehensive income		643	(552)	1,961	102
Balance, end of period		(23,545)	(22,602)	(23,545)	(22,602)
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax		(39,514)	(34,668)	(42,035)	(38,440)
Adjustment to opening balance, pretax						803		(26)
Adjusted balance, beginning of period, pretax						(41,232)		(38,466)
Other comprehensive income (loss), pretax		408	954	2,126	4,752
Balance, end of period, pretax		(39,106)	(33,714)	(39,106)	(33,714)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		9,101	8,042	9,654	9,096
Adjustment to opening balance, tax effect						(299)		11
Adjusted balance, beginning of period, tax effect						(9,355)		(9,107)
Other comprehensive income (loss), tax effect		106	(443)	(148)	(1,508)
Balance, end of period, tax effect		9,207	7,599	9,207	7,599
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(30,413)	(26,626)	(32,381)	(29,344)
Adjustment to opening balance	[1]					504		(15)
Adjusted balance, beginning of period						$ (31,877)		$ (29,359)
Total other comprehensive income		514	511	1,978	3,244
Balance, end of period		$ (29,899)	$ (26,115)	$ (29,899)	$ (26,115)
Accounting Standards Update 2016-01
Change in accumulated other comprehensive income (loss), after-tax
Reclassification from AOCI							$ 15
Retained earnings							$ 15

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