CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets
Cash and due from banks	$ 24,448	$ 23,645
Cash and due from banks—intercompany	0	0
Deposits with banks	181,445	164,460
Deposits with banks—intercompany	0	0
Federal funds sold and securities borrowed and purchased under agreements to resell (including $162,116 and $147,701 as of March 31, 2019 and December 31, 2018, respectively, at fair value)	264,495	270,684
Federal funds sold and resale agreements—intercompany	0	0
Trading account assets	286,511	256,117
Trading account assets—intercompany	0	0
Investments	349,281	358,607
Loans, net of unearned income	682,346	684,196
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	(12,329)	(12,315)	$ (12,354)	$ (12,355)
Total loans, net	670,017	671,881
Advances to subsidiaries	0	0
Investments in subsidiaries	0	0
Other assets	182,216	171,989
Other assets—intercompany	0	0
Total assets	1,958,413	1,917,383
Liabilities and equity
Deposits	1,030,355	1,013,170
Deposits—intercompany	0	0
Federal funds purchased and securities loaned and sold under agreements to repurchase (including $46,241 and $44,510 as of March 31, 2019 and December 31, 2018, respectively, at fair value)	190,372	177,768
Federal funds purchased and securities loaned and sold—intercompany	0	0
Trading account liabilities	136,392	144,305
Trading account liabilities—intercompany	0	0
Short-term borrowings	39,322	32,346
Short-term borrowings—intercompany	0	0
Long-term debt	243,566	231,999
Long-term debt—intercompany	0	0
Advances from subsidiaries	0	0
Other liabilities	121,391	120,721
Other liabilities—intercompany	0	0
Stockholders’ equity	197,015	197,074	$ 202,866
Total liabilities and equity	1,958,413	1,917,383
Other	110,483	109,273
Citigroup parent company
Liabilities and equity
Long-term debt	149,830	143,767
Citibank, N.A.
Liabilities and equity
Other	47,400	34,700
Reportable legal entities | Citigroup parent company
Assets
Cash and due from banks	1	1
Cash and due from banks—intercompany	11	19
Deposits with banks	0	0
Deposits with banks—intercompany	3,000	3,000
Federal funds sold and securities borrowed and purchased under agreements to resell (including $162,116 and $147,701 as of March 31, 2019 and December 31, 2018, respectively, at fair value)	0	0
Federal funds sold and resale agreements—intercompany	0	0
Trading account assets	296	302
Trading account assets—intercompany	825	627
Investments	5	7
Loans, net of unearned income	0	0
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	0	0
Total loans, net	0	0
Advances to subsidiaries	142,884	143,119
Investments in subsidiaries	201,016	205,337
Other assets	11,957	9,861
Other assets—intercompany	3,734	3,037
Total assets	363,729	365,310
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Federal funds purchased and securities loaned and sold under agreements to repurchase (including $46,241 and $44,510 as of March 31, 2019 and December 31, 2018, respectively, at fair value)	0	0
Federal funds purchased and securities loaned and sold—intercompany	0	0
Trading account liabilities	14	1
Trading account liabilities—intercompany	1,863	410
Short-term borrowings	234	207
Short-term borrowings—intercompany	0	0
Long-term debt	149,830	143,768
Long-term debt—intercompany	0	0
Advances from subsidiaries	11,634	21,471
Other liabilities	3,308	3,010
Other liabilities—intercompany	594	223
Stockholders’ equity	196,252	196,220
Total liabilities and equity	363,729	365,310
Reportable legal entities | CGMHI
Assets
Cash and due from banks	963	689
Cash and due from banks—intercompany	3,953	3,545
Deposits with banks	5,287	4,915
Deposits with banks—intercompany	6,235	6,528
Federal funds sold and securities borrowed and purchased under agreements to resell (including $162,116 and $147,701 as of March 31, 2019 and December 31, 2018, respectively, at fair value)	210,012	212,720
Federal funds sold and resale agreements—intercompany	16,034	20,074
Trading account assets	163,582	146,233
Trading account assets—intercompany	1,770	1,728
Investments	241	224
Loans, net of unearned income	1,731	1,292
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	0	0
Total loans, net	1,731	1,292
Advances to subsidiaries	0	0
Investments in subsidiaries	0	0
Other assets	63,919	59,734
Other assets—intercompany	50,591	44,255
Total assets	524,318	501,937
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Federal funds purchased and securities loaned and sold under agreements to repurchase (including $46,241 and $44,510 as of March 31, 2019 and December 31, 2018, respectively, at fair value)	163,595	155,830
Federal funds purchased and securities loaned and sold—intercompany	28,561	21,109
Trading account liabilities	94,159	95,571
Trading account liabilities—intercompany	1,919	1,398
Short-term borrowings	6,485	3,656
Short-term borrowings—intercompany	20,468	11,343
Long-term debt	30,542	25,986
Long-term debt—intercompany	73,094	73,884
Advances from subsidiaries	0	0
Other liabilities	62,484	66,732
Other liabilities—intercompany	10,443	13,763
Stockholders’ equity	32,568	32,665
Total liabilities and equity	524,318	501,937
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Assets
Cash and due from banks	23,484	22,955
Cash and due from banks—intercompany	(3,964)	(3,564)
Deposits with banks	176,158	159,545
Deposits with banks—intercompany	(9,235)	(9,528)
Federal funds sold and securities borrowed and purchased under agreements to resell (including $162,116 and $147,701 as of March 31, 2019 and December 31, 2018, respectively, at fair value)	54,483	57,964
Federal funds sold and resale agreements—intercompany	(16,034)	(20,074)
Trading account assets	122,633	109,582
Trading account assets—intercompany	(2,595)	(2,355)
Investments	349,035	358,376
Loans, net of unearned income	680,615	682,904
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	(12,329)	(12,315)
Total loans, net	668,286	670,589
Advances to subsidiaries	(142,884)	(143,119)
Investments in subsidiaries	0	0
Other assets	106,340	102,394
Other assets—intercompany	(54,325)	(47,292)
Total assets	1,271,382	1,255,473
Liabilities and equity
Deposits	1,030,355	1,013,170
Deposits—intercompany	0	0
Federal funds purchased and securities loaned and sold under agreements to repurchase (including $46,241 and $44,510 as of March 31, 2019 and December 31, 2018, respectively, at fair value)	26,777	21,938
Federal funds purchased and securities loaned and sold—intercompany	(28,561)	(21,109)
Trading account liabilities	42,219	48,733
Trading account liabilities—intercompany	(3,782)	(1,808)
Short-term borrowings	32,603	28,483
Short-term borrowings—intercompany	(20,468)	(11,343)
Long-term debt	63,194	62,245
Long-term debt—intercompany	(73,094)	(73,884)
Advances from subsidiaries	(11,634)	(21,471)
Other liabilities	55,599	50,979
Other liabilities—intercompany	(11,037)	(13,986)
Stockholders’ equity	169,211	173,526
Total liabilities and equity	1,271,382	1,255,473
Consolidating adjustments
Assets
Cash and due from banks	0	0
Cash and due from banks—intercompany	0	0
Deposits with banks	0	0
Deposits with banks—intercompany	0	0
Federal funds sold and securities borrowed and purchased under agreements to resell (including $162,116 and $147,701 as of March 31, 2019 and December 31, 2018, respectively, at fair value)	0	0
Federal funds sold and resale agreements—intercompany	0	0
Trading account assets	0	0
Trading account assets—intercompany	0	0
Investments	0	0
Loans, net of unearned income	0	0
Loans, net of unearned income—intercompany	0	0
Allowance for loan losses	0	0
Total loans, net	0	0
Advances to subsidiaries	0	0
Investments in subsidiaries	(201,016)	(205,337)
Other assets	0	0
Other assets—intercompany	0	0
Total assets	(201,016)	(205,337)
Liabilities and equity
Deposits	0	0
Deposits—intercompany	0	0
Federal funds purchased and securities loaned and sold under agreements to repurchase (including $46,241 and $44,510 as of March 31, 2019 and December 31, 2018, respectively, at fair value)	0	0
Federal funds purchased and securities loaned and sold—intercompany	0	0
Trading account liabilities	0	0
Trading account liabilities—intercompany	0	0
Short-term borrowings	0	0
Short-term borrowings—intercompany	0	0
Long-term debt	0	0
Long-term debt—intercompany	0	0
Advances from subsidiaries	0	0
Other liabilities	0	0
Other liabilities—intercompany	0	0
Stockholders’ equity	(201,016)	(205,337)
Total liabilities and equity	(201,016)	(205,337)
Up to 30 days | Citibank, N.A.
Liabilities and equity
Placements with term of less than 30 days	$ 35,800	$ 22,400