CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses on sales of investments	$ (213)	$ (287)	$ (626)	$ (673)
Income from continuing operations before income taxes	(6,003)	(5,620)	(17,662)	(16,377)
Provision (benefits) for income taxes	1,866	1,733	5,524	4,935
Income (loss) from continuing operations	(4,137)	(3,887)	(12,138)	(11,442)
Realized gains (losses) on investment securities
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	(126)	(162)	(368)	(252)
Realized gains (losses) on investment securities \| (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses on sales of investments	(213)	(287)	(626)	(673)
OTTI gross impairment losses	15	32	47	283
Income from continuing operations before income taxes	(198)	(255)	(579)	(390)
Provision (benefits) for income taxes	72	93	211	138
Income (loss) from continuing operations	(126)	(162)	(368)	(252)
Debt valuation adjustment (DVA)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	2	(3)	(8)	(6)
Debt valuation adjustment (DVA) \| (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses on sales of investments	3	(5)	(13)	(10)
Income from continuing operations before income taxes	3	(5)	(13)	(10)
Provision (benefits) for income taxes	(1)	2	5	4
Income (loss) from continuing operations	2	(3)	(8)	(6)
Cash flow hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	35	53	64	115
Cash flow hedges \| (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	55	85	102	185
Provision (benefits) for income taxes	(20)	(32)	(38)	(70)
Income (loss) from continuing operations	35	53	64	115
Cash flow hedges \| (Gain) loss reclassified from AOCI \| Interest rate contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	48	39	94	96
Cash flow hedges \| (Gain) loss reclassified from AOCI \| Foreign exchange contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	7	46	8	89
Pension liability adjustments
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	65	71	183	186
Total tax effect	(23)	(31)	(66)	(72)
Total amounts reclassified out of AOCI, after-tax	42	40	117	114
Prior service cost (benefit)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	(10)	(10)	(32)	(31)
Net actuarial loss
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	70	73	203	208
Curtailment/settlement impact
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	5	8	12	9
Foreign currency translation adjustment
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	0	0	(147)	0
Foreign currency translation adjustment \| (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Income from continuing operations before income taxes	0		(232)
Provision (benefits) for income taxes	0		85
Income (loss) from continuing operations	0	0	(147)	0
Citigroup's accumulated other comprehensive income (loss)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	(75)	(104)	(539)	(29)
Total tax effect	28	32	197	0
Total amounts reclassified out of AOCI, after-tax	$ (47)	$ (72)	$ (342)	$ (29)

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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