CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		$ 30,066	$ (3,461)
Cash flows from investing activities of continuing operations
Purchases of investments		(129,054)	(151,362)
Proceeds from sales of investments		52,170	89,724
Proceeds from maturities of investments		82,940	67,166
Change in loans		(16,131)	(41,569)
Proceeds from sales and securitizations of loans		4,021	7,019
Proceeds from significant disposals	[1]	314	3,411
Change in federal funds sold and resales		(48,462)	(15,795)
Changes in investments and advances—intercompany		0	0
Other investing activities		(2,361)	(2,054)
Net cash used in investing activities of continuing operations		(56,563)	(43,460)
Cash flows from financing activities of continuing operations
Dividends paid		(3,616)	(2,639)
Redemption of preferred stock		(218)	0
Treasury stock acquired		(9,848)	(9,071)
Proceeds (repayments) from issuance of long-term debt, net		5,826	22,508
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		45,354	34,632
Change in federal funds purchased and repos		19,638	19,461
Change in short-term borrowings		(10,681)	7,448
Net change in short-term borrowings and other advances—intercompany		0	0
Capital contributions from (to) parent		0	0
Other financing activities		(479)	(402)
Net cash provided by financing activities of continuing operations		45,976	71,937
Effect of exchange rate changes on cash and due from banks		(709)	599
Change in cash and due from banks and deposits with banks	[2]	18,770	25,615
Cash, due from banks and deposits with banks at beginning of period	[2]	180,516	160,494
Cash, due from banks and deposits with banks at end of period	[2]	199,286	186,109
Cash and due from banks				$ 25,727	$ 23,775	$ 22,604
Deposits with banks				173,559	156,741	163,505
Cash, due from banks and deposits with banks at end of period	[2]	180,516	160,494	199,286	180,516	186,109
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		3,261	2,714
Cash paid during the period for interest		16,278	11,604
Non-cash investing activities
Transfers to loans HFS from loans		3,300	3,800
Transfers to OREO and other repossessed assets		94	85
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		12,581	5,712
Cash flows from investing activities of continuing operations
Purchases of investments		(7,955)	0
Proceeds from sales of investments		7,634	132
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	0
Change in federal funds sold and resales		0	0
Changes in investments and advances—intercompany		(7,769)	13,269
Other investing activities		214	0
Net cash used in investing activities of continuing operations		(7,876)	13,401
Cash flows from financing activities of continuing operations
Dividends paid		(3,616)	(2,639)
Redemption of preferred stock		(218)
Treasury stock acquired		(9,848)	(9,071)
Proceeds (repayments) from issuance of long-term debt, net		(883)	6,665
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		0	0
Change in federal funds purchased and repos		0	0
Change in short-term borrowings		32	44
Net change in short-term borrowings and other advances—intercompany		2,312	(23,342)
Capital contributions from (to) parent		0	0
Other financing activities		(479)	(402)
Net cash provided by financing activities of continuing operations		(12,700)	(28,745)
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		(7,995)	(9,632)
Cash, due from banks and deposits with banks at beginning of period		11,013	20,811
Cash, due from banks and deposits with banks at end of period		3,018	11,179
Cash and due from banks				18		179
Deposits with banks				3,000		11,000
Cash, due from banks and deposits with banks at end of period		11,013	20,811	3,018	11,013	11,179
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		873	(772)
Cash paid during the period for interest		2,870	3,319
Non-cash investing activities
Transfers to loans HFS from loans		0	0
Transfers to OREO and other repossessed assets		0	0
Reportable legal entities | CGMHI
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		16,232	(15,236)
Cash flows from investing activities of continuing operations
Purchases of investments		(18)	0
Proceeds from sales of investments		3	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	0
Change in federal funds sold and resales		(47,943)	(8,840)
Changes in investments and advances—intercompany		(2,338)	(5,439)
Other investing activities		(41)	0
Net cash used in investing activities of continuing operations		(50,337)	(14,279)
Cash flows from financing activities of continuing operations
Dividends paid		0	0
Redemption of preferred stock		0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		7,538	4,385
Proceeds (repayments) from issuance of long-term debt—intercompany, net		5,048	(1,300)
Change in deposits		0	0
Change in federal funds purchased and repos		35,804	6,910
Change in short-term borrowings		790	1,865
Net change in short-term borrowings and other advances—intercompany		(14,771)	6,573
Capital contributions from (to) parent		(663)	(60)
Other financing activities		0	0
Net cash provided by financing activities of continuing operations		33,746	18,373
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		(359)	(11,142)
Cash, due from banks and deposits with banks at beginning of period		12,695	25,118
Cash, due from banks and deposits with banks at end of period		12,336	13,976
Cash and due from banks				2,648		4,519
Deposits with banks				9,688		9,457
Cash, due from banks and deposits with banks at end of period		12,695	25,118	12,336	12,695	13,976
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		138	470
Cash paid during the period for interest		6,045	3,175
Non-cash investing activities
Transfers to loans HFS from loans		0	0
Transfers to OREO and other repossessed assets		0	0
Reportable legal entities | Other Citigroup subsidiaries and eliminations
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		1,253	6,063
Cash flows from investing activities of continuing operations
Purchases of investments		(121,081)	(151,362)
Proceeds from sales of investments		44,533	89,592
Proceeds from maturities of investments		82,940	67,166
Change in loans		(16,131)	(41,569)
Proceeds from sales and securitizations of loans		4,021	7,019
Proceeds from significant disposals		314	3,411
Change in federal funds sold and resales		(519)	(6,955)
Changes in investments and advances—intercompany		10,107	(7,830)
Other investing activities		(2,534)	(2,054)
Net cash used in investing activities of continuing operations		1,650	(42,582)
Cash flows from financing activities of continuing operations
Dividends paid		0	0
Redemption of preferred stock		0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		(829)	11,458
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(5,048)	1,300
Change in deposits		45,354	34,632
Change in federal funds purchased and repos		(16,166)	12,551
Change in short-term borrowings		(11,503)	5,539
Net change in short-term borrowings and other advances—intercompany		12,459	16,769
Capital contributions from (to) parent		663	60
Other financing activities		0	0
Net cash provided by financing activities of continuing operations		24,930	82,309
Effect of exchange rate changes on cash and due from banks		(709)	599
Change in cash and due from banks and deposits with banks		27,124	46,389
Cash, due from banks and deposits with banks at beginning of period		156,808	114,565
Cash, due from banks and deposits with banks at end of period		183,932	160,954
Cash and due from banks				23,061		17,906
Deposits with banks				160,871		143,048
Cash, due from banks and deposits with banks at end of period		156,808	114,565	183,932	156,808	160,954
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes		2,250	3,016
Cash paid during the period for interest		7,363	5,110
Non-cash investing activities
Transfers to loans HFS from loans		3,300	3,800
Transfers to OREO and other repossessed assets		94	85
Consolidating adjustments
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by 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	0
Change in federal funds sold and resales		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
Redemption of preferred stock		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 federal funds purchased and repos		0	0
Change in short-term borrowings		0	0
Net change in short-term borrowings and other advances—intercompany		0	0
Capital contributions from (to) 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, 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 year 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	$ 0

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

[2] Includes the impact of ASU 2016-18, Restricted Cash. See Notes 1 and 22 to the Consolidated Financial Statements.