CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses on sales of investments	$ (170)	$ (192)
Income from continuing operations before income taxes	(6,090)	(5,981)
Provision (benefits) for income taxes	1,441	1,863
Income (loss) from continuing operations	(4,649)	(4,118)
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	(109)	(114)
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	(170)	(192)
Gross impairment losses	27	12
Income from continuing operations before income taxes	(143)	(180)
Provision (benefits) for income taxes	34	66
Income (loss) from continuing operations	(109)	(114)
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	27	(5)
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	35	(8)
Income from continuing operations before income taxes	35	(8)
Provision (benefits) for income taxes	(8)	3
Income (loss) from continuing operations	27	(5)
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	29	(41)
Provision (benefits) for income taxes	(8)	15
Income (loss) from continuing operations	21	(26)
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	31	(44)
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	(2)	3
Pension liability adjustments
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	62	57
Total tax effect	(15)	(20)
Total amounts reclassified out of AOCI, after-tax	47	37
Prior service cost (benefit)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	(11)	(10)
Net actuarial loss
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	69	67
Curtailment/settlement impact
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	4	0
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	(147)
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	(147)
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	(17)	(404)
Total tax effect	3	149
Total amounts reclassified out of AOCI, after-tax	$ (14)	$ (255)

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 35
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