CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		$ 197,074
Balance, end of period		197,015	$ 202,866
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax		(44,082)	(41,228)
Adjustment to opening balance, pretax					$ (4)
Adjusted balance, beginning of period, pretax					(41,232)
Other comprehensive income (loss), pretax		1,178	(287)
Balance, end of period, pretax		(42,904)	(41,519)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect		6,912	6,560
Adjustment to opening balance, tax effect					1
Adjusted balance, beginning of period, tax effect					6,561
Other comprehensive income (loss), tax effect		(316)	339
Balance, end of period, tax effect		6,596	6,900
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(37,170)	(34,668)
Adjustment to opening balance	[1]			$ 0	(3)
Adjusted balance, beginning of period				(37,170)	(34,671)
Total other comprehensive income		862	52
Balance, end of period		(36,308)	(34,619)
Net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		1,500	(1,380)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(365)	322
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(2,250)	(1,158)
Adjustment to opening balance				0	(3)
Adjusted balance, beginning of period				(2,250)	(1,161)
Total other comprehensive income		1,135	(1,058)
Balance, end of period		(1,115)	(2,219)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(725)	167
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		154	(39)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		192	(921)
Adjustment to opening balance				0	0
Adjusted balance, beginning of period				192	(921)
Total other comprehensive income		(571)	128
Balance, end of period		(379)	(793)
Hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(57)	0
Adjustment to opening balance					0
Adjusted balance, beginning of period				(57)	0
Total other comprehensive income		18	(4)
Balance, end of period		(39)	(4)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		(68)	91
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		4	(3)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(6,257)	(6,183)
Adjustment to opening balance				0	0
Adjusted balance, beginning of period				(6,257)	(6,183)
Total other comprehensive income		(64)	88
Balance, end of period		(6,321)	(6,095)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		69	1,130
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(11)	(10)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period		(28,070)	(25,708)
Adjustment to opening balance				0	0
Adjusted balance, beginning of period				$ (28,070)	$ (25,708)
Total other comprehensive income		58	1,120
Balance, end of period		(28,012)	(24,588)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		378	(290)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(92)	68
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		286	(222)
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax		24	(5)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect		(6)	1
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income		$ 18	$ (4)

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