CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		$ 201,672
Balance, end of period		202,866	$ 228,997
Net unrealized gains (losses) on investment securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(1,380)	346
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		322	(126)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(1,158)	(799)
Adjustment to opening balance				$ (3)	$ 504
Adjusted balance, beginning of period				(1,161)	(295)
Total other comprehensive income		(1,058)	220
Balance, end of period		(2,219)	(75)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		167	(95)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(39)	35
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(921)	(352)
Adjustment to opening balance				0	0
Adjusted balance, beginning of period				(921)	(352)
Total other comprehensive income		128	(60)
Balance, end of period		(793)	(412)
Cash flow hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(290)	1
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		68	(3)
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		(222)	(2)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		91	(2)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(3)	(10)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(6,183)	(5,164)
Adjustment to opening balance				0	0
Adjusted balance, beginning of period				(6,183)	(5,164)
Total other comprehensive income		88	(12)
Balance, end of period		(6,095)	(5,176)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		1,130	1,468
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(10)	(150)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(25,708)	(25,506)
Adjustment to opening balance				0	0
Adjusted balance, beginning of period				(25,708)	(25,506)
Total other comprehensive income		1,120	1,318
Balance, end of period		(24,588)	(24,188)
Excluded component of fair value hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(5)	0
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		1	0
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		0	0
Adjustment to opening balance				0	0
Adjusted balance, beginning of period				0	0
Total other comprehensive income		(4)	0
Balance, end of period		(4)	0
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax		(41,228)	(42,035)
Adjustment to opening balance, pretax				(4)	803
Adjusted balance, beginning of period, pretax				(41,232)	(41,232)
Other comprehensive income (loss), pretax		(287)	1,718
Balance, end of period, pretax		(41,519)	(39,514)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		6,560	9,654
Adjustment to opening balance, tax effect				1	(299)
Adjusted balance, beginning of period, tax effect				6,561	9,355
Other comprehensive income (loss), tax effect		339	(254)
Balance, end of period, tax effect		6,900	9,101
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(34,668)	(32,381)
Adjustment to opening balance	[1]			(3)	504
Adjusted balance, beginning of period				$ (34,671)	$ (31,877)
Total other comprehensive income		52	1,464
Balance, end of period		$ (34,619)	$ (30,413)

[1] See Note 1 to the Consolidated Financial Statements for additional details.