CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		$ 23,563	$ (19,208)
Cash flows from investing activities of continuing operations
Purchases of investments		(201,567)	(207,701)
Proceeds from sales of investments		66,477	86,191
Proceeds from maturities of investments		75,195	53,909
Change in loans		(3,088)	7,943
Proceeds from sales and securitizations of loans		869	826
Change in securities borrowed and purchased under agreements to resell		(14,335)	(31,595)
Changes in investments and advances—intercompany		0	0
Other investing activities		(1,662)	(1,262)
Net cash used in investing activities of continuing operations		(78,111)	(91,689)
Cash flows from financing activities of continuing operations
Dividends paid		(2,663)	(2,679)
Issuance of preferred stock		2,300	1,500
Redemption of preferred stock		(3,785)	(1,500)
Treasury stock acquired		(4,381)	(2,925)
Proceeds (repayments) from issuance of long-term debt, net		(383)	26,174
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		29,610	163,070
Change in securities loaned and sold under agreements to repurchase		22,292	49,383
Change in short-term borrowings		1,948	(4,893)
Net change in short-term borrowings and other advances—intercompany		0	0
Other financing activities		(324)	(407)
Net cash provided by financing activities of continuing operations		44,614	227,723
Effect of exchange rate changes on cash and due from banks		(443)	(972)
Change in cash, due from banks and deposits with banks		(10,377)	115,854
Cash, due from banks and deposits with banks at beginning of period		309,615	193,919
Cash, due from banks and deposits with banks at end of period		299,238	309,773
Cash and due from banks (including segregated cash and other deposits)		27,117	22,889	$ 26,349
Deposits with banks, net of allowance		272,121	286,884	283,266
Cash, due from banks and deposits with banks at end of period		299,238	309,773	309,615
Supplemental disclosure of cash flow information for continuing operations
Cash paid (received) during the period for income taxes		2,176	2,543
Cash paid during the period for interest		4,545	8,751
Non-cash investing activities
Transfers to loans HFS from loans	[1]	961	1,036
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		1,429	2,857
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
Change in securities borrowed and purchased under agreements to resell		0	0
Changes in investments and advances—intercompany		(2,424)	(7,371)
Other investing activities		0	0
Net cash used in investing activities of continuing operations		(2,424)	(7,371)
Cash flows from financing activities of continuing operations
Dividends paid		(2,663)	(2,679)
Issuance of preferred stock		2,300	1,500
Redemption of preferred stock		(3,785)	(1,500)
Treasury stock acquired		(4,381)	(2,925)
Proceeds (repayments) from issuance of long-term debt, net		7,576	17,353
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		772	(6,826)
Other financing activities		(324)	(407)
Net cash provided by financing activities of continuing operations		(505)	4,516
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash, due from banks and deposits with banks		(1,500)	2
Cash, due from banks and deposits with banks at beginning of period		4,516	3,021
Cash, due from banks and deposits with banks at end of period		3,016	3,023
Cash and due from banks (including segregated cash and other deposits)		16	23
Deposits with banks, net of allowance		3,000	3,000
Cash, due from banks and deposits with banks at end of period		3,016	3,023	4,516
Supplemental disclosure of cash flow information for continuing operations
Cash paid (received) during the period for income taxes		(1,437)	39
Cash paid during the period for interest		1,287	1,757
Non-cash investing activities
Transfers to loans HFS from loans		0	0
Reportable legal entities | CGMHI
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		5,912	(53,782)
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
Change in securities borrowed and purchased under agreements to resell		(14,084)	(29,475)
Changes in investments and advances—intercompany		(7,360)	(4,890)
Other investing activities		(15)	0
Net cash used in investing activities of continuing operations		(21,459)	(34,365)
Cash flows from financing activities of continuing operations
Dividends paid		(187)	0
Issuance of preferred stock		0	0
Redemption of preferred stock		0	0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		8,446	8,907
Proceeds (repayments) from issuance of long-term debt—intercompany, net		11,040	6,815
Change in deposits		0	0
Change in securities loaned and sold under agreements to repurchase		(9,152)	68,650
Change in short-term borrowings		3,358	1,074
Net change in short-term borrowings and other advances—intercompany		4,885	3,035
Other financing activities		0	(118)
Net cash provided by financing activities of continuing operations		18,390	88,363
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash, due from banks and deposits with banks		2,843	216
Cash, due from banks and deposits with banks at beginning of period		20,112	16,441
Cash, due from banks and deposits with banks at end of period		22,955	16,657
Cash and due from banks (including segregated cash and other deposits)		6,642	3,728
Deposits with banks, net of allowance		16,313	12,929
Cash, due from banks and deposits with banks at end of period		22,955	16,657	20,112
Supplemental disclosure of cash flow information for continuing operations
Cash paid (received) during the period for income taxes		649	174
Cash paid during the period for interest		1,197	3,006
Non-cash investing activities
Transfers to loans HFS from loans		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		16,222	31,717
Cash flows from investing activities of continuing operations
Purchases of investments		(201,567)	(207,701)
Proceeds from sales of investments		66,477	86,191
Proceeds from maturities of investments		75,195	53,909
Change in loans		(3,088)	7,943
Proceeds from sales and securitizations of loans		869	826
Change in securities borrowed and purchased under agreements to resell		(251)	(2,120)
Changes in investments and advances—intercompany		9,784	12,261
Other investing activities		(1,647)	(1,262)
Net cash used in investing activities of continuing operations		(54,228)	(49,953)
Cash flows from financing activities of continuing operations
Dividends paid		187	0
Issuance of preferred stock		0	0
Redemption of preferred stock		0	0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		(16,405)	(86)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(11,040)	(6,815)
Change in deposits		29,610	163,070
Change in securities loaned and sold under agreements to repurchase		31,444	(19,267)
Change in short-term borrowings		(1,410)	(5,967)
Net change in short-term borrowings and other advances—intercompany		(5,657)	3,791
Other financing activities		0	118
Net cash provided by financing activities of continuing operations		26,729	134,844
Effect of exchange rate changes on cash and due from banks		(443)	(972)
Change in cash, due from banks and deposits with banks		(11,720)	115,636
Cash, due from banks and deposits with banks at beginning of period		284,987	174,457
Cash, due from banks and deposits with banks at end of period		273,267	290,093
Cash and due from banks (including segregated cash and other deposits)		20,459	19,138
Deposits with banks, net of allowance		252,808	270,955
Cash, due from banks and deposits with banks at end of period		273,267	290,093	284,987
Supplemental disclosure of cash flow information for continuing operations
Cash paid (received) during the period for income taxes		2,964	2,330
Cash paid during the period for interest		2,061	3,988
Non-cash investing activities
Transfers to loans HFS from loans		961	1,036
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
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	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
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, 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 (including segregated cash and other deposits)		0	0
Deposits with banks, net of allowance		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 (received) 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

[1] Operating and finance lease right-of-use assets and lease liabilities represent non-cash investing and financing activities, respectively, and are not included in the non-cash investing activities presented here. See Note 22 to the Consolidated Financial Statements for more information and balances as of June 30, 2021.