CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized (gains) losses on sales of investments									$ (1,756)	$ (1,474)	$ (421)
Income from continuing operations before income taxes	$ (5,441)	$ (3,954)	$ (1,109)	$ (3,128)	$ (5,702)	$ (6,022)	$ (6,165)	$ (6,012)
Tax effect	1,116	777	52	580	703	1,079	1,373	1,275	2,525	4,430	5,357
Income (loss) from continuing operations	$ (4,325)	$ (3,177)	$ (1,057)	$ (2,548)	$ (4,999)	$ (4,943)	$ (4,792)	$ (4,737)	(11,107)	(19,471)	(18,088)
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									(1,252)	(1,080)	(223)
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									(1,756)	(1,474)	(421)
Gross impairment losses									109	23	125
Income from continuing operations before income taxes									(1,647)	(1,451)	(296)
Tax effect									395	371	73
Income (loss) from continuing operations									(1,252)	(1,080)	(223)
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									15	15	32
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									20	20	41
Tax effect									(5)	(5)	(9)
Income (loss) from continuing operations									15	15	32
Cash flow hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax									(557)	302	105
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									(730)	391	318
Tax effect									173	(89)	(213)
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									(734)	384	301
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									4	7	17
Benefit plans
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax									309	275	220
Total tax effect									(77)	(69)	(54)
Total amounts reclassified out of AOCI, after-tax									232	206	166
Prior service cost (benefit)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax									(5)	(12)	(34)
Net actuarial loss
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax									322	286	248
Curtailment/settlement impact
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, pretax									(8)	1	6
Excluded component of fair value 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
Excluded component of fair value 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									0	0	0
Tax effect									0	0	0
Total amounts reclassified out of AOCI, after-tax									0	0	0
Foreign currency translation adjustment (CTA), net of hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax									0	0	245
Foreign currency translation adjustment (CTA), net of 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									0	0	34
Tax effect									0	0	211
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									(2,048)	(765)	317
Total tax effect									486	208	8
Total amounts reclassified out of AOCI, after-tax									$ (1,562)	$ (557)	$ 325