CHANGES IN ACCUMULATED OTHER COMPREHENSIVE INCOME (LOSS) (AOCI) - Schedule of Pre-Tax and After-Tax (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period			$ 197,074
Balance, end of period	$ 198,110	$ 200,968	198,110	$ 200,968
Citigroup's accumulated other comprehensive income (loss)
Change in accumulated other comprehensive income (loss), pretax
Balance, beginning of period, pretax	(42,904)	(41,519)	(44,082)	(41,228)
Adjustment to opening balance, pretax								$ (4)
Adjusted balance, beginning of period, pretax								(41,232)
Other comprehensive income (loss), pretax	1,432	(2,888)	2,610	(3,175)
Balance, end of period, pretax	(41,472)	(44,407)	(41,472)	(44,407)
Change in accumulated other comprehensive income (loss), tax effect
Balance, beginning of period, tax effect	6,596	6,900	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	(327)	13	(643)	352
Balance, end of period, tax effect	6,269	6,913	6,269	6,913
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(36,308)	(34,619)	(37,170)	(34,668)
Adjustment to opening balance					$ 0	$ 0	$ 0	(3)	$ (3)
Adjusted balance, beginning of period					$ (36,308)	$ (37,170)	$ (34,619)	(34,671)	$ (34,671)
Total other comprehensive income	1,105	(2,875)	1,967	(2,823)
Balance, end of period	(35,203)	(37,494)	(35,203)	(37,494)
Net unrealized gains (losses) on debt securities
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	936	(671)	2,436	(2,051)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(233)	173	(598)	495
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(1,115)	(2,219)	(2,250)	(1,158)
Adjustment to opening balance								(3)
Adjusted balance, beginning of period								(1,161)
Total other comprehensive income	703	(498)	1,838	(1,556)
Balance, end of period	(412)	(2,717)	(412)	(2,717)
Debt valuation adjustment (DVA)
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	3	418	(722)	585
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	0	(100)	154	(139)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(379)	(793)	192	(921)
Adjustment to opening balance								0
Adjusted balance, beginning of period								(921)
Total other comprehensive income	3	318	(568)	446
Balance, end of period	(376)	(475)	(376)	(475)
Hedges
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(39)	(4)	(57)	0
Adjustment to opening balance								0
Adjusted balance, beginning of period								0
Total other comprehensive income	44	(28)	62	(32)
Balance, end of period	5	(32)	5	(32)
Benefit plans
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	(329)	403	(397)	494
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	76	(102)	80	(105)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(6,321)	(6,095)	(6,257)	(6,183)
Adjustment to opening balance								0
Adjusted balance, beginning of period								(6,183)
Total other comprehensive income	(253)	301	(317)	389
Balance, end of period	(6,574)	(5,794)	(6,574)	(5,794)
Foreign currency translation adjustment
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	83	(2,869)	152	(1,739)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	8	2	(3)	(8)
Change in accumulated other comprehensive income (loss), after-tax
Balance, beginning of period	(28,012)	(24,588)	(28,070)	(25,708)
Adjustment to opening balance								0
Adjusted balance, beginning of period								$ (25,708)
Total other comprehensive income	91	(2,867)	149	(1,747)
Balance, end of period	(27,921)	(27,455)	(27,921)	(27,455)
Cash flow hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	680	(132)	1,058	(422)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(163)	31	(255)	99
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	517	(101)	803	(323)
Fair value hedges | Hedges
Change in accumulated other comprehensive income (loss), pretax
Other comprehensive income (loss), pretax	59	(37)	83	(42)
Change in accumulated other comprehensive income (loss), tax effect
Other comprehensive income (loss), tax effect	(15)	9	(21)	10
Change in accumulated other comprehensive income (loss), after-tax
Total other comprehensive income	$ 44	$ (28)	$ 62	$ (32)