CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		$ 6,956	$ (3,052)
Cash flows from investing activities of continuing operations
Purchases of investments		(41,030)	(41,584)
Proceeds from sales of investments		20,688	29,456
Proceeds from maturities of investments		21,509	24,006
Change in loans		(8,717)	(7,953)
Proceeds from sales and securitizations of loans		1,654	3,191
Proceeds from significant disposals	[1]	0	2,732
Change in federal funds sold and resales		(25,409)	(6,116)
Changes in investments and advances—intercompany		0	0
Other investing activities		(819)	(607)
Net cash provided by (used in) investing activities of continuing operations		(32,124)	3,125
Cash flows from financing activities of continuing operations
Dividends paid		(1,095)	(744)
Redemption of preferred stock		(97)	0
Treasury stock acquired		(2,378)	(1,858)
Proceeds from issuance of long-term debt, net		2,887	(282)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		41,397	20,584
Change in federal funds purchased and repos		15,482	6,409
Change in short-term borrowings		(8,358)	(4,574)
Net change in short-term borrowings and other advances—intercompany		0	0
Capital contributions from (to) parent		0
Other financing activities		(475)	(397)
Net cash provided by financing activities of continuing operations		47,363	19,138
Effect of exchange rate changes on cash and due from banks		(7)	340
Change in cash and due from banks and deposits with banks	[2]	22,188	19,551
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]	202,704	180,045
Cash and due from banks				$ 21,850	$ 23,775	$ 22,272
Deposits with banks				180,854	156,741	157,773
Cash, due from banks and deposits with banks at end of period	[2]	180,516	160,494	202,704	180,516	180,045
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		738	913
Cash paid during the period for interest		4,586	3,250
Non-cash investing activities
Transfers to loans HFS from loans		900	2,800
Transfers to OREO and other repossessed assets		26	30
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		5,268	(652)
Cash flows from investing activities of continuing operations
Purchases of investments		(7,955)	0
Proceeds from sales of investments		0	116
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 federal funds sold and resales		0	0
Changes in investments and advances—intercompany		(1,463)	(569)
Other investing activities		(729)	0
Net cash provided by (used in) investing activities of continuing operations		(10,147)	(453)
Cash flows from financing activities of continuing operations
Dividends paid		(1,095)	(744)
Redemption of preferred stock		(97)
Treasury stock acquired		(2,378)	(1,858)
Proceeds from issuance of long-term debt, net		699	(1,454)
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		14	(14,901)
Capital contributions from (to) parent		0
Other financing activities		(261)	(397)
Net cash provided by financing activities of continuing operations		(3,118)	(19,354)
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		(7,997)	(20,459)
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		3,016	352
Cash and due from banks				16		352
Deposits with banks				3,000		0
Cash, due from banks and deposits with banks at end of period		11,013	20,811	3,016	11,013	352
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		(266)	(139)
Cash paid during the period for interest		883	1,153
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		7,046	(3,404)
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 federal funds sold and resales		(22,167)	(2,623)
Changes in investments and advances—intercompany		(3,603)	(5,007)
Other investing activities		(9)	0
Net cash provided by (used in) investing activities of continuing operations		(25,779)	(7,630)
Cash flows from financing activities of continuing operations
Dividends paid		0	0
Redemption of preferred stock		0
Treasury stock acquired		0	0
Proceeds from issuance of long-term debt, net		2,004	5,175
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(412)	(12,506)
Change in deposits		0	0
Change in federal funds purchased and repos		11,359	1,266
Change in short-term borrowings		(409)	605
Net change in short-term borrowings and other advances—intercompany		8,226	8,938
Capital contributions from (to) parent		(585)
Other financing activities		0	0
Net cash provided by financing activities of continuing operations		20,183	3,478
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		1,450	(7,556)
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		14,145	17,562
Cash and due from banks				5,648		3,647
Deposits with banks				8,497		13,915
Cash, due from banks and deposits with banks at end of period		12,695	25,118	14,145	12,695	17,562
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		29	64
Cash paid during the period for interest		1,627	822
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		(5,358)	1,004
Cash flows from investing activities of continuing operations
Purchases of investments		(33,075)	(41,584)
Proceeds from sales of investments		20,688	29,340
Proceeds from maturities of investments		21,509	24,006
Change in loans		(8,717)	(7,953)
Proceeds from sales and securitizations of loans		1,654	3,191
Proceeds from significant disposals			2,732
Change in federal funds sold and resales		(3,242)	(3,493)
Changes in investments and advances—intercompany		5,066	5,576
Other investing activities		(81)	(607)
Net cash provided by (used in) investing activities of continuing operations		3,802	11,208
Cash flows from financing activities of continuing operations
Dividends paid		0	0
Redemption of preferred stock		0
Treasury stock acquired		0	0
Proceeds from issuance of long-term debt, net		184	(4,003)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		412	12,506
Change in deposits		41,397	20,584
Change in federal funds purchased and repos		4,123	5,143
Change in short-term borrowings		(7,949)	(5,179)
Net change in short-term borrowings and other advances—intercompany		(8,240)	5,963
Capital contributions from (to) parent		585
Other financing activities		(214)	0
Net cash provided by financing activities of continuing operations		30,298	35,014
Effect of exchange rate changes on cash and due from banks		(7)	340
Change in cash and due from banks and deposits with banks		28,735	47,566
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		185,543	162,131
Cash and due from banks				16,186		18,273
Deposits with banks				169,357		143,858
Cash, due from banks and deposits with banks at end of period		156,808	114,565	185,543	156,808	162,131
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		975	988
Cash paid during the period for interest		2,076	1,275
Non-cash investing activities
Transfers to loans HFS from loans		900	2,800
Transfers to OREO and other repossessed assets		26	30
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 federal funds sold and resales		0	0
Changes in investments and advances—intercompany		0	0
Other investing activities		0	0
Net cash provided by (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 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
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 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.

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