CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Reclassification out of AOCI (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2025	Mar. 31, 2024
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized gains and (losses)		$ 121	$ 115
Impairment losses on investments		(58)	(30)
Subtotal, pretax		5,448	4,544
Tax effect		(1,340)	(1,136)
Income (loss) from continuing operations		4,108	3,408
Subtotal, pretax		(152)	477
Other Noninterest Expense	[1]	(1,349)	(1,820)
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, after-tax		119	231
Citigroup's accumulated other comprehensive income (loss) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax		119	231
Total tax effect		(30)	(74)
Total amounts reclassified out of AOCI, pretax		149	305
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		(86)	(76)
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 and (losses)		(121)	(115)
Impairment losses on investments		3	14
Subtotal, pretax		(118)	(101)
Tax effect		32	25
Income (loss) from continuing operations		(86)	(76)
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		4	10
Debt valuation adjustment (DVA) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Realized gains and (losses)		5	13
Tax effect		(1)	(3)
Income (loss) from continuing operations		4	10
Cash flow hedges
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax		143	260
Cash flow hedges | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Tax effect		(46)	(83)
Subtotal, pretax		189	343
Total amounts reclassified out of AOCI, after-tax		143	260
Cash flow hedges | (Gain) loss reclassified from AOCI | Interest rate contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Subtotal, pretax		189	342
Cash flow hedges | (Gain) loss reclassified from AOCI | Foreign exchange contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Subtotal, pretax		0	1
Benefit plan adjustments
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax		45	47
Benefit plan adjustments | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense		60	63
Total amounts reclassified out of AOCI, after-tax		45	47
Total tax effect		(15)	(16)
Prior service cost (benefit) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense		(4)	(5)
Net actuarial loss | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense		64	68
Curtailment/settlement impact | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Other Noninterest Expense		0	0
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		1	(10)
Excluded component of fair value hedges | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Subtotal, pretax		1	(13)
Tax effect		0	3
Total amounts reclassified out of AOCI, after-tax		1	(10)
Long-duration insurance contracts
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax		0	0
Long-duration insurance contracts | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Subtotal, pretax		0	0
Tax effect		0	0
Total amounts reclassified out of AOCI, after-tax		0	0
Foreign currency translation adjustment (CTA)
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Total amounts reclassified out of AOCI, after-tax		12	0
Foreign currency translation adjustment (CTA) | (Gain) loss reclassified from AOCI
Pretax and after-tax amounts reclassified out of accumulated other comprehensive income (loss)
Subtotal, pretax		12	0
Tax effect		0	0
Total amounts reclassified out of AOCI, after-tax		$ 12	$ 0

[1]     Effective January 1, 2025, certain transaction processing fees paid by Citi, primarily to credit card networks, which were previously presented within Other operating expenses, are presented as contra-revenue within Commissions and fees reported in Non-interest revenue. Prior periods were conformed to reflect this change in presentation.