CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		$ (35,358)	$ 30,066
Cash flows from investing activities of continuing operations
Purchases of investments		(196,733)	(112,554)
Proceeds from sales of investments		96,400	52,170
Proceeds from maturities of investments		91,656	66,440
Change in loans		(11,518)	(16,131)
Proceeds from sales and securitizations of loans		2,717	4,021
Proceeds from significant disposals	[1]	0	314
Change in securities borrowed and purchased under agreements to resell		9,559	(48,462)
Changes in investments and advances—intercompany		0	0
Other investing activities		(4,173)	(2,361)
Net cash used in investing activities of continuing operations		(12,092)	(56,563)
Cash flows from financing activities of continuing operations
Dividends paid		(4,048)	(3,616)
Issuance of preferred stock		1,496	0
Redemption of preferred stock		(480)	(218)
Treasury stock acquired		(12,495)	(9,848)
Proceeds (repayments) from issuance of long-term debt, net		2,276	5,826
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		74,599	45,354
Change in securities loaned and sold under agreements to repurchase		17,279	19,638
Change in short-term borrowings		2,884	(10,681)
Net change in short-term borrowings and other advances—intercompany		0	0
Capital contributions from parent		0	0
Other financing activities		(360)	(479)
Net cash provided by financing activities of continuing operations		81,151	45,976
Effect of exchange rate changes on cash and due from banks		(1,363)	(709)
Change in cash and due from banks and deposits with banks		32,338	18,770
Cash, due from banks and deposits with banks at beginning of period		188,105	180,516
Cash, due from banks and deposits with banks at end of period		220,443	199,286
Cash and due from banks				$ 24,086	$ 23,645	$ 25,727
Deposits with banks				196,357	164,460	173,559
Cash and due from banks and deposits with banks at end of period		188,105	199,286	220,443	188,105	199,286
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		3,735	3,261
Cash paid during the period for interest		22,343	16,278
Non-cash investing activities
Transfers to loans HFS from loans		4,400	3,300
Transfers to OREO and other repossessed assets			94
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		23,879	12,581
Cash flows from investing activities of continuing operations
Purchases of investments		0	(7,955)
Proceeds from sales of investments		4	7,634
Proceeds from maturities of investments		0	0
Change in loans		0	0
Proceeds from sales and securitizations of loans		0	0
Proceeds from significant disposals			0
Change in securities borrowed and purchased under agreements to resell		0	0
Changes in investments and advances—intercompany		(2,045)	(7,769)
Other investing activities		0	214
Net cash used in investing activities of continuing operations		(2,041)	(7,876)
Cash flows from financing activities of continuing operations
Dividends paid		(4,048)	(3,616)
Issuance of preferred stock		1,496
Redemption of preferred stock		(480)	(218)
Treasury stock acquired		(12,495)	(9,848)
Proceeds (repayments) from issuance of long-term debt, net		(1,122)	(883)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		0	0
Change in securities loaned and sold under agreements to repurchase		0	0
Change in short-term borrowings		0	32
Net change in short-term borrowings and other advances—intercompany		(4,834)	2,312
Capital contributions from parent		0	0
Other financing activities		(360)	(479)
Net cash provided by financing activities of continuing operations		(21,843)	(12,700)
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		(5)	(7,995)
Cash, due from banks and deposits with banks at beginning of period		3,020	11,013
Cash, due from banks and deposits with banks at end of period		3,015	3,018
Cash and due from banks				15		18
Deposits with banks				3,000		3,000
Cash and due from banks and deposits with banks at end of period		3,020	11,013	3,015	3,020	3,018
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		(274)	873
Cash paid during the period for interest		3,107	2,870
Non-cash investing activities
Transfers to loans HFS from loans		0	0
Transfers to OREO and other repossessed assets			0
Reportable legal entities | CGMHI
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		(51,748)	16,232
Cash flows from investing activities of continuing operations
Purchases of investments		0	(18)
Proceeds from sales of investments		0	3
Proceeds from maturities of investments		0	0
Change in loans		0	0
Proceeds from sales and securitizations of loans		0	0
Proceeds from significant disposals			0
Change in securities borrowed and purchased under agreements to resell		8,914	(47,943)
Changes in investments and advances—intercompany		(6,204)	(2,338)
Other investing activities		(44)	(41)
Net cash used in investing activities of continuing operations		2,666	(50,337)
Cash flows from financing activities of continuing operations
Dividends paid		(155)	0
Issuance of preferred stock		0
Redemption of preferred stock		0	0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		10,136	7,538
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(5,683)	5,048
Change in deposits		0	0
Change in securities loaned and sold under agreements to repurchase		25,598	35,804
Change in short-term borrowings		5,855	790
Net change in short-term borrowings and other advances—intercompany		15,211	(14,771)
Capital contributions from parent		(74)	(663)
Other financing activities		0	0
Net cash provided by financing activities of continuing operations		50,888	33,746
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		1,806	(359)
Cash, due from banks and deposits with banks at beginning of period		15,677	12,695
Cash, due from banks and deposits with banks at end of period		17,483	12,336
Cash and due from banks				5,501		2,648
Deposits with banks				11,982		9,688
Cash and due from banks and deposits with banks at end of period		15,677	12,695	17,483	15,677	12,336
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		281	138
Cash paid during the period for interest		8,893	6,045
Non-cash investing activities
Transfers to loans HFS from loans		0	0
Transfers to OREO and other repossessed assets			0
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		(7,489)	1,253
Cash flows from investing activities of continuing operations
Purchases of investments		(196,733)	(104,581)
Proceeds from sales of investments		96,396	44,533
Proceeds from maturities of investments		91,656	66,440
Change in loans		(11,518)	(16,131)
Proceeds from sales and securitizations of loans		2,717	4,021
Proceeds from significant disposals			314
Change in securities borrowed and purchased under agreements to resell		645	(519)
Changes in investments and advances—intercompany		8,249	10,107
Other investing activities		(4,129)	(2,534)
Net cash used in investing activities of continuing operations		(12,717)	1,650
Cash flows from financing activities of continuing operations
Dividends paid		155	0
Issuance of preferred stock		0
Redemption of preferred stock		0	0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		(6,738)	(829)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		5,683	(5,048)
Change in deposits		74,599	45,354
Change in securities loaned and sold under agreements to repurchase		(8,319)	(16,166)
Change in short-term borrowings		(2,971)	(11,503)
Net change in short-term borrowings and other advances—intercompany		(10,377)	12,459
Capital contributions from parent		74	663
Other financing activities		0	0
Net cash provided by financing activities of continuing operations		52,106	24,930
Effect of exchange rate changes on cash and due from banks		(1,363)	(709)
Change in cash and due from banks and deposits with banks		30,537	27,124
Cash, due from banks and deposits with banks at beginning of period		169,408	156,808
Cash, due from banks and deposits with banks at end of period		199,945	183,932
Cash and due from banks				18,570		23,061
Deposits with banks				181,375		160,871
Cash and due from banks and deposits with banks at end of period		169,408	156,808	199,945	169,408	183,932
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		3,728	2,250
Cash paid during the period for interest		10,343	7,363
Non-cash investing activities
Transfers to loans HFS from loans		4,400	3,300
Transfers to OREO and other repossessed assets			94
Consolidating adjustments
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		0	0
Cash flows from investing activities of continuing operations
Purchases of investments		0	0
Proceeds from sales of investments		0	0
Proceeds from maturities of investments		0	0
Change in loans		0	0
Proceeds from sales and securitizations of loans		0	0
Proceeds from significant disposals			0
Change in securities borrowed and purchased under agreements to resell		0	0
Changes in investments and advances—intercompany		0	0
Other investing activities		0	0
Net cash used in investing activities of continuing operations		0	0
Cash flows from financing activities of continuing operations
Dividends paid		0	0
Issuance of preferred stock		0
Redemption of preferred stock		0	0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		0	0
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		0	0
Change in securities loaned and sold under agreements to repurchase		0	0
Change in short-term borrowings		0	0
Net change in short-term borrowings and other advances—intercompany		0	0
Capital contributions from parent		0	0
Other financing activities		0	0
Net cash provided by financing activities of continuing operations		0	0
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		0	0
Cash, due from banks and deposits with banks at beginning of period		0	0
Cash, due from banks and deposits with banks at end of period		0	0
Cash and due from banks				0		0
Deposits with banks				0		0
Cash and due from banks and deposits with banks at end of period		0	0	$ 0	$ 0	$ 0
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		0	0
Cash paid during the period for interest		0	0
Non-cash investing activities
Transfers to loans HFS from loans		$ 0	0
Transfers to OREO and other repossessed assets			$ 0

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