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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 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		$ 228,622	$ 232,690	228,622	$ 232,690
Reclassification from AOCI		29,891		29,891		$ 32,381
Retained earnings		155,174		155,174		146,477
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(107)	(686)	194	4,020
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		41	254	(67)	(1,491)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(102)	2,054	(799)	(907)
Adjustment to opening balance						504		$ 0
Adjusted balance, beginning of period						(295)		(907)
Total other comprehensive income		(66)	(432)	127	2,529
Balance, end of period		(168)	1,622	(168)	1,622
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(195)	(319)	(422)	8
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		72	119	155	(3)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(496)	190	(352)	0
Adjustment to opening balance						0		(15)
Adjusted balance, beginning of period						(352)		(15)
Total other comprehensive income		(123)	(200)	(267)	5
Balance, end of period		(619)	(10)	(619)	(10)
Cash flow hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		12	(131)	198	607
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(4)	48	(75)	(222)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(445)	(149)	(560)	(617)
Adjustment to opening balance						0		0
Adjusted balance, beginning of period						(560)		(617)
Total other comprehensive income		8	(83)	123	385
Balance, end of period		(437)	(232)	(437)	(232)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(45)	11	(266)	(747)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		16	1	90	267
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(5,311)	(5,608)	(5,164)	(5,116)
Adjustment to opening balance						0		0
Adjusted balance, beginning of period						(5,164)		(5,116)
Total other comprehensive income		(29)	12	(176)	(480)
Balance, end of period		(5,340)	(5,596)	(5,340)	(5,596)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		285	(313)	2,372	(574)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(67)	(62)	(193)	301
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(23,545)	(22,602)	(25,506)	(22,704)
Adjustment to opening balance						0		0
Adjusted balance, beginning of period						(25,506)		(22,704)
Total other comprehensive income		218	(375)	2,179	(273)
Balance, end of period		(23,327)	(22,977)	(23,327)	(22,977)
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax		(39,106)	(33,714)	(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		(50)	(1,438)	2,076	3,314
Balance, end of period, pretax		(39,156)	(35,152)	(39,156)	(35,152)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		9,207	7,599	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		58	360	(90)	(1,148)
Balance, end of period, tax effect		9,265	7,959	9,265	7,959
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(29,899)	(26,115)	(32,381)	(29,344)
Adjustment to opening balance	[1]					504		(15)
Adjusted balance, beginning of period						$ (31,877)		$ (29,359)
Total other comprehensive income		8	(1,078)	1,986	2,166
Balance, end of period		$ (29,891)	$ (27,193)	$ (29,891)	$ (27,193)
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.