CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		$ (3,305)	$ 28,334
Cash flows from investing activities of continuing operations
Purchases of investments		(151,362)	(155,804)
Proceeds from sales of investments		89,724	99,172
Proceeds from maturities of investments		67,166	52,607
Change in deposits with banks		(26,054)	(20,374)
Change in loans		(41,569)	(42,163)
Proceeds from sales and securitizations of loans		7,019	12,676
Proceeds from significant disposals	[1]	3,411	265
Change in federal funds sold and resales		(15,795)	(16,370)
Changes in investments and advances—intercompany		0	0
Other investing activities		(2,210)	(1,625)
Net cash used in investing activities of continuing operations		(69,670)	(71,616)
Cash flows from financing activities of continuing operations
Dividends paid		(2,639)	(1,517)
Issuance of preferred stock		0	2,498
Treasury stock acquired		(9,071)	(5,167)
Proceeds (repayments) from issuance of long-term debt, net		22,508	3,003
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		34,632	32,365
Change in federal funds purchased and repos		19,461	6,628
Change in short-term borrowings		7,448	8,448
Net change in short-term borrowings and other advances—intercompany		0	0
Capital contributions from parent		0	0
Other financing activities		(402)	(313)
Net cash provided by financing activities of continuing operations		71,937	45,945
Effect of exchange rate changes on cash and cash equivalents		599	(144)
Change in cash and due from banks		(439)	2,519
Cash and due from banks at beginning of period		23,043	20,900
Cash and due from banks at end of period		22,604	23,419
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		2,714	2,855
Cash paid during the period for interest		11,604	9,760
Non-cash investing activities
Transfers to loans HFS from loans		3,800	8,600
Transfers to OREO and other repossessed assets		85	138
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		15,381	16,685
Cash flows from investing activities of continuing operations
Purchases of investments		0	0
Proceeds from sales of investments		132	229
Proceeds from maturities of investments		0	61
Change in deposits with banks		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		13,269	(14,378)
Other investing activities		0	2,962
Net cash used in investing activities of continuing operations		13,401	(11,126)
Cash flows from financing activities of continuing operations
Dividends paid		(2,639)	(1,517)
Issuance of preferred stock			2,498
Treasury stock acquired		(9,071)	(5,167)
Proceeds (repayments) from issuance of long-term debt, net		6,665	1,613
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		44	(163)
Net change in short-term borrowings and other advances—intercompany		(23,342)	(2,503)
Capital contributions from parent		0	0
Other financing activities		(402)	(313)
Net cash provided by financing activities of continuing operations		(28,745)	(5,552)
Effect of exchange rate changes on cash and cash equivalents		0	0
Change in cash and due from banks		37	7
Cash and due from banks at beginning of period		142	124
Cash and due from banks at end of period		179	131
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		(772)	(265)
Cash paid during the period for interest		3,319	3,402
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 (used in) operating activities of continuing operations		(15,237)	5,285
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 deposits with banks		10,972	(1,464)
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		(8,840)	(12,398)
Changes in investments and advances—intercompany		(5,439)	(23)
Other investing activities		0	0
Net cash used in investing activities of continuing operations		(3,307)	(13,885)
Cash flows from financing activities of continuing operations
Dividends paid		0	0
Issuance of preferred stock			0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		4,385	4,196
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(1,300)	(12,533)
Change in deposits		0	0
Change in federal funds purchased and repos		6,910	12,251
Change in short-term borrowings		1,865	1,251
Net change in short-term borrowings and other advances—intercompany		6,573	(726)
Capital contributions from parent		(60)	5,000
Other financing activities		0	0
Net cash provided by financing activities of continuing operations		18,373	9,439
Effect of exchange rate changes on cash and cash equivalents		0	0
Change in cash and due from banks		(171)	839
Cash and due from banks at beginning of period		4,690	1,995
Cash and due from banks at end of period		4,519	2,834
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		470	81
Cash paid during the period for interest		3,175	2,378
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 (used in) operating activities of continuing operations		(3,449)	6,364
Cash flows from investing activities of continuing operations
Purchases of investments		(151,362)	(155,804)
Proceeds from sales of investments		89,592	98,943
Proceeds from maturities of investments		67,166	52,546
Change in deposits with banks		(37,026)	(18,910)
Change in loans		(41,569)	(42,163)
Proceeds from sales and securitizations of loans		7,019	12,676
Proceeds from significant disposals		3,411	265
Change in federal funds sold and resales		(6,955)	(3,972)
Changes in investments and advances—intercompany		(7,830)	14,401
Other investing activities		(2,210)	(4,587)
Net cash used in investing activities of continuing operations		(79,764)	(46,605)
Cash flows from financing activities of continuing operations
Dividends paid		0	0
Issuance of preferred stock			0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		11,458	(2,806)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		1,300	12,533
Change in deposits		34,632	32,365
Change in federal funds purchased and repos		12,551	(5,623)
Change in short-term borrowings		5,539	7,360
Net change in short-term borrowings and other advances—intercompany		16,769	3,229
Capital contributions from parent		60	(5,000)
Other financing activities		0	0
Net cash provided by financing activities of continuing operations		82,309	42,058
Effect of exchange rate changes on cash and cash equivalents		599	(144)
Change in cash and due from banks		(305)	1,673
Cash and due from banks at beginning of period		18,211	18,781
Cash and due from banks at end of period		17,906	20,454
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		3,016	3,039
Cash paid during the period for interest		5,110	3,980
Non-cash investing activities
Transfers to loans HFS from loans		3,800	8,600
Transfers to OREO and other repossessed assets		85	138
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 deposits with banks		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
Issuance 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 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 cash equivalents		0	0
Change in cash and due from banks		0	0
Cash and due from banks at beginning of period		0	0
Cash and due from banks at end of period		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	$ 0

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