CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses on sales of investments	$ (468)	$ (102)	$ (598)	$ (272)
Income from continuing operations before income taxes	(6,165)	(5,945)	(12,177)	(12,035)
Provision (benefits) for income taxes	1,373	1,444	2,648	2,885
Income (loss) from continuing operations	(4,792)	(4,501)	(9,529)	(9,150)
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	(347)	(65)	(438)	(173)
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	(468)	(102)	(598)	(272)
Gross impairment losses	2	15	5	43
Income from continuing operations before income taxes	(466)	(87)	(593)	(229)
Provision (benefits) for income taxes	119	22	155	56
Income (loss) from continuing operations	(347)	(65)		(173)
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	17	2	21	29
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	22	2	27	37
Income from continuing operations before income taxes	22	2	27	37
Provision (benefits) for income taxes	(5)	0	(6)	(8)
Income (loss) from continuing operations	17	2		29
Hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax	0	0	0	0
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	136	(86)	268	(57)
Provision (benefits) for income taxes	(33)	21	(65)	13
Income (loss) from continuing operations		(65)		(44)
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	134	(82)	264	(51)
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	(4)	4	(6)
Pension liability adjustments
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	69	55	130	117
Total tax effect	(17)	(15)	(32)	(30)
Total amounts reclassified out of AOCI, after-tax	52	40	98	87
Prior service cost (benefit)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	(2)	(11)	(6)	(22)
Net actuarial loss
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	69	64	134	133
Curtailment/settlement impact
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax	2	2	2	6
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	0	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	0	0	0
Provision (benefits) for income taxes	0	0	0	0
Income (loss) from continuing operations		0		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	(239)	(116)	(168)	(132)
Total tax effect	64	28	52	31
Total amounts reclassified out of AOCI, after-tax	$ (175)	$ (88)	$ (116)	$ (101)

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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