CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		$ 36,952	$ (8,774)	$ 53,733
Cash flows from investing activities of continuing operations
Purchases of investments		(186,987)	(185,740)	(211,402)
Proceeds from sales of investments	[1]	61,491	107,368	132,183
Proceeds from maturities of investments		118,104	84,369	65,525
Change in loans		(29,002)	(58,062)	(39,761)
Proceeds from sales and securitizations of loans		4,549	8,365	18,140
Proceeds from significant disposals	[2]	314	3,411	265
Change in federal funds sold and resales		(38,206)	4,335	(17,138)
Payments due to transfers of net liabilities associated with significant disposals				0
Changes in investments and advances—intercompany		0	0	0
Other investing activities		(3,381)	(2,797)	(1,947)
Net cash used in investing activities of continuing operations		(73,118)	(38,751)	(54,135)
Cash flows from financing activities of continuing operations
Dividends paid		(5,020)	(3,797)	(2,287)
Issuance (redemption) of preferred stock		(793)	0	2,498
Treasury stock acquired		(14,433)	(14,541)	(9,290)
Proceeds (repayments) from issuance of long-term debt, net		2,523	26,974	8,346
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0	0
Change in deposits		53,348	30,416	24,394
Change in federal funds purchased and repos		21,491	14,456	(4,675)
Change in short-term borrowings		(12,106)	13,751	9,622
Net change in short-term borrowings and other advances—intercompany		0	0	0
Capital contributions from parent		0	0
Other financing activities		(482)	(405)	(316)
Net cash provided by financing activities of continuing operations		44,528	66,854	28,292
Effect of exchange rate changes on cash and cash equivalents		(773)	693	(493)
Change in cash and due from banks and deposits with banks	[3]	7,589	20,022	27,397
Cash and due from banks and deposits with banks at beginning of period		180,516	160,494	133,097
Cash and due from banks and deposits with banks at end of period		188,105	180,516	160,494
Cash and due from banks		23,645	23,775	23,043
Deposits with banks		164,460	156,741	137,451
Cash, due from banks and deposits with banks at end of period	[3]	188,105	180,516	160,494	$ 133,097
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		4,313	2,083	4,359
Cash paid during the year for interest		22,963	15,675	12,067
Non-cash investing activities
Transfers to loans HFS from loans		4,200	5,900	13,900
Transfers to OREO and other repossessed assets		151	113	165
Consolidating adjustments
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		0	0	0
Cash flows from investing activities of continuing operations
Purchases of investments		0	0	0
Proceeds from sales of investments		0	0	0
Proceeds from maturities of investments		0	0	0
Change in loans		0	0	0
Proceeds from sales and securitizations of loans		0	0	0
Proceeds from significant disposals		0	0	0
Change in federal funds sold and resales		0	0	0
Payments due to transfers of net liabilities associated with significant disposals				0
Changes in investments and advances—intercompany		0	0	0
Other investing activities		0	0	0
Net cash used in investing activities of continuing operations		0	0	0
Cash flows from financing activities of continuing operations
Dividends paid		0	0	0
Issuance (redemption) of preferred stock		0	0	0
Treasury stock acquired		0	0	0
Proceeds (repayments) from issuance of long-term debt, net		0	0	0
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0	0
Change in deposits		0	0	0
Change in federal funds purchased and repos		0	0	0
Change in short-term borrowings		0	0	0
Net change in short-term borrowings and other advances—intercompany		0	0	0
Capital contributions from parent		0	0
Other financing activities		0	0	0
Net cash provided by financing activities of continuing operations		0	0	0
Effect of exchange rate changes on cash and cash equivalents		0	0	0
Change in cash and due from banks and deposits with banks		0	0	0
Cash and due from banks and deposits with banks at beginning of period		0	0	0
Cash and due from banks and deposits with banks at end of period		0	0	0
Cash and due from banks		0	0	0
Deposits with banks		0	0	0
Cash, due from banks and deposits with banks at end of period		0	0	0
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		0	0	0
Cash paid during the year for interest		0	0	0
Non-cash investing activities
Transfers to loans HFS from loans		0	0	0
Transfers to OREO and other repossessed assets		0	0	0
Citigroup parent company | Reportable legal entities
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		21,314	25,270	11,605
Cash flows from investing activities of continuing operations
Purchases of investments		(7,955)	0	0
Proceeds from sales of investments		7,634	132	3,024
Proceeds from maturities of investments		0	0	234
Change in loans		0	0	0
Proceeds from sales and securitizations of loans		0	0	0
Proceeds from significant disposals		0	0	0
Change in federal funds sold and resales		0	0	0
Payments due to transfers of net liabilities associated with significant disposals				0
Changes in investments and advances—intercompany		(5,566)	(899)	(18,083)
Other investing activities		556	0	0
Net cash used in investing activities of continuing operations		(5,331)	(767)	(14,825)
Cash flows from financing activities of continuing operations
Dividends paid		(5,020)	(3,797)	(2,287)
Issuance (redemption) of preferred stock		(793)	0	2,498
Treasury stock acquired		(14,433)	(14,541)	(9,290)
Proceeds (repayments) from issuance of long-term debt, net		(5,099)	6,544	7,005
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0	0
Change in deposits		0	0	0
Change in federal funds purchased and repos		0	0	0
Change in short-term borrowings		32	49	(164)
Net change in short-term borrowings and other advances—intercompany		1,819	(22,152)	4,620
Capital contributions from parent		0	0
Other financing activities		(482)	(405)	(316)
Net cash provided by financing activities of continuing operations		(23,976)	(34,302)	2,066
Effect of exchange rate changes on cash and cash equivalents		0	0	0
Change in cash and due from banks and deposits with banks		(7,993)	(9,799)	(1,154)
Cash and due from banks and deposits with banks at beginning of period		11,013	20,812	21,966
Cash and due from banks and deposits with banks at end of period		3,020	11,013	20,812
Cash and due from banks		20	13	142
Deposits with banks		3,000	11,000	20,670
Cash, due from banks and deposits with banks at end of period		3,020	11,013	20,812
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		(783)	(3,730)	351
Cash paid during the year for interest		3,854	4,151	4,397
Non-cash investing activities
Transfers to loans HFS from loans		0	0	0
Transfers to OREO and other repossessed assets		0	0	0
CGMHI | Reportable legal entities
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		13,287	(33,365)	20,610
Cash flows from investing activities of continuing operations
Purchases of investments		(18)	(14)	(14)
Proceeds from sales of investments		3	18	0
Proceeds from maturities of investments		0	0	0
Change in loans		0	0	0
Proceeds from sales and securitizations of loans		0	0	0
Proceeds from significant disposals		0	0	0
Change in federal funds sold and resales		(34,018)	9,731	(15,294)
Payments due to transfers of net liabilities associated with significant disposals				0
Changes in investments and advances—intercompany		(832)	9,755	(5,574)
Other investing activities		(59)	(24)	57
Net cash used in investing activities of continuing operations		(34,924)	19,466	(20,825)
Cash flows from financing activities of continuing operations
Dividends paid		0	0	0
Issuance (redemption) of preferred stock		0	0	0
Treasury stock acquired		0	0	0
Proceeds (repayments) from issuance of long-term debt, net		10,278	4,909	5,916
Proceeds (repayments) from issuance of long-term debt—intercompany, net		10,708	(2,031)	(9,453)
Change in deposits		0	0	0
Change in federal funds purchased and repos		23,454	5,748	3,236
Change in short-term borrowings		88	2,212	1,168
Net change in short-term borrowings and other advances—intercompany		(19,111)	(8,615)	680
Capital contributions from parent		(798)	(748)
Other financing activities		0	0	0
Net cash provided by financing activities of continuing operations		24,619	1,475	6,557
Effect of exchange rate changes on cash and cash equivalents		0	0	0
Change in cash and due from banks and deposits with banks		2,982	(12,424)	6,342
Cash and due from banks and deposits with banks at beginning of period		12,695	25,119	18,777
Cash and due from banks and deposits with banks at end of period		15,677	12,695	25,119
Cash and due from banks		4,234	4,128	4,690
Deposits with banks		11,443	8,567	20,429
Cash, due from banks and deposits with banks at end of period		15,677	12,695	25,119
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		458	678	92
Cash paid during the year for interest		8,671	4,513	3,115
Non-cash investing activities
Transfers to loans HFS from loans		0	0	0
Transfers to OREO and other repossessed assets		0	0	0
Other Citigroup subsidiaries and eliminations | Reportable legal entities
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		2,351	(679)	21,518
Cash flows from investing activities of continuing operations
Purchases of investments		(179,014)	(185,726)	(211,388)
Proceeds from sales of investments		53,854	107,218	129,159
Proceeds from maturities of investments		118,104	84,369	65,291
Change in loans		(29,002)	(58,062)	(39,761)
Proceeds from sales and securitizations of loans		4,549	8,365	18,140
Proceeds from significant disposals		314	3,411	265
Change in federal funds sold and resales		(4,188)	(5,396)	(1,844)
Payments due to transfers of net liabilities associated with significant disposals				0
Changes in investments and advances—intercompany		6,398	(8,856)	23,657
Other investing activities		(3,878)	(2,773)	(2,004)
Net cash used in investing activities of continuing operations		(32,863)	(57,450)	(18,485)
Cash flows from financing activities of continuing operations
Dividends paid		0	0	0
Issuance (redemption) of preferred stock		0	0	0
Treasury stock acquired		0	0	0
Proceeds (repayments) from issuance of long-term debt, net		(2,656)	15,521	(4,575)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(10,708)	2,031	9,453
Change in deposits		53,348	30,416	24,394
Change in federal funds purchased and repos		(1,963)	8,708	(7,911)
Change in short-term borrowings		(12,226)	11,490	8,618
Net change in short-term borrowings and other advances—intercompany		17,292	30,767	(5,300)
Capital contributions from parent		798	748
Other financing activities		0	0	0
Net cash provided by financing activities of continuing operations		43,885	99,681	19,669
Effect of exchange rate changes on cash and cash equivalents		(773)	693	(493)
Change in cash and due from banks and deposits with banks		12,600	42,245	22,209
Cash and due from banks and deposits with banks at beginning of period		156,808	114,563	92,354
Cash and due from banks and deposits with banks at end of period		169,408	156,808	114,563
Cash and due from banks		19,391	19,634	18,211
Deposits with banks		150,017	137,174	96,352
Cash, due from banks and deposits with banks at end of period		169,408	156,808	114,563
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		4,638	5,135	3,916
Cash paid during the year for interest		10,438	7,011	4,555
Non-cash investing activities
Transfers to loans HFS from loans		4,200	5,900	13,900
Transfers to OREO and other repossessed assets		$ 151	$ 113	$ 165

[1] Proceeds for 2016 include approximately $3.3 billion from the sale of Citi’s investment in China Guangfa Bank.

[2] See Note 2 to the Consolidated Financial Statements for further information on significant disposals.

[3] Includes the impact of ASU 2016-18, Restricted Cash. See Notes 1 and 26 to the Consolidated Financial Statements.