CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015	Dec. 31, 2014
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period				$ 223,092
Balance, end of period		$ 232,690	$ 222,137	232,690	$ 222,137
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(686)	821	4,020	353
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		254	(310)	(1,491)	(186)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		2,054	(287)	(907)	57
Adjustment to opening balance						$ 0
Adjusted balance, beginning of period						(907)
Change, net of taxes		(432)	511	2,529	167
Balance, end of period		1,622	224	1,622	224
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(319)		8
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		119		(3)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		190		0
Adjustment to opening balance						(15)
Adjusted balance, beginning of period						(15)
Change, net of taxes		(200)		5
Balance, end of period		(10)		(10)
Cash flow hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(131)	322	607	596
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		48	(133)	(222)	(229)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(149)	(731)	(617)	(909)
Adjustment to opening balance						0
Adjusted balance, beginning of period						(617)
Change, net of taxes		(83)	189	385	367
Balance, end of period		(232)	(542)	(232)	(542)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		11	(545)	(747)	144
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		1	185	267	(16)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(5,608)	(4,671)	(5,116)	(5,159)
Adjustment to opening balance						0
Adjusted balance, beginning of period						(5,116)
Change, net of taxes		12	(360)	(480)	128
Balance, end of period		(5,596)	(5,031)	(5,596)	(5,031)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(313)	(2,792)	(574)	(5,375)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(62)	299	301	672
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(22,602)	(19,415)	(22,704)	(17,205)
Adjustment to opening balance						0
Adjusted balance, beginning of period						(22,704)
Change, net of taxes		(375)	(2,493)	(273)	(4,703)
Balance, end of period		(22,977)	(21,908)	(22,977)	(21,908)
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax		(33,714)	(33,148)	(38,440)	(31,060)
Adjustment to opening balance, pretax						(26)
Adjusted balance, beginning of period, pretax						(38,466)
Other comprehensive income (loss), pretax		(1,438)	(2,194)	3,314	(4,282)
Balance, end of period, pretax		(35,152)	(35,342)	(35,152)	(35,342)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		7,599	8,044	9,096	7,844
Adjustment to opening balance, tax effect						11
Adjusted balance, beginning of period, tax effect						9,107
Other comprehensive income (loss), tax effect		360	41	(1,148)	241
Balance, end of period, tax effect		7,959	8,085	7,959	8,085
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(26,115)	(25,104)	(29,344)	(23,216)
Adjustment to opening balance	[1]					(15)	$ 0
Adjusted balance, beginning of period						$ (29,359)	$ (23,216)
Change, net of taxes		(1,078)	(2,153)	2,166	(4,041)
Balance, end of period		$ (27,193)	$ (27,257)	$ (27,193)	$ (27,257)

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