CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		$ 59,185	$ (22,254)
Cash flows from investing activities of continuing operations
Purchases of investments		(277,874)	(276,084)
Proceeds from sales of investments		96,203	130,237
Proceeds from maturities of investments		107,361	78,476
Change in loans		6,613	23,488
Proceeds from sales and securitizations of loans		1,134	924
Change in securities borrowed and purchased under agreements to resell		(42,984)	(38,036)
Changes in investments and advances—intercompany		0	0
Other investing activities		(2,522)	(2,205)
Net cash used in investing activities of continuing operations		(112,069)	(83,200)
Cash flows from financing activities of continuing operations
Dividends paid		(3,959)	(4,024)
Issuance of preferred stock		2,300	1,500
Redemption of preferred stock		(3,785)	(1,500)
Treasury stock acquired		(7,448)	(2,925)
Proceeds (repayments) from issuance of long-term debt, net		(2,511)	18,078
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		73,769	192,033
Change in securities loaned and sold under agreements to repurchase		9,659	40,888
Change in short-term borrowings		169	(7,610)
Net change in short-term borrowings and other advances—intercompany		0	0
Capital contributions from (to) parent		0
Other financing activities		(328)	(408)
Net cash provided by financing activities of continuing operations		67,866	236,032
Effect of exchange rate changes on cash and due from banks		(789)	(802)
Change in cash, due from banks and deposits with banks		14,193	129,776
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		323,808	323,695
Cash and due from banks (including segregated cash and other deposits)		28,906	25,308	$ 26,349
Deposits with banks, net of allowance		294,902	298,387	283,266
Cash, due from banks and deposits with banks at end of period		323,808	323,695	309,615
Supplemental disclosure of cash flow information for continuing operations
Cash paid (received) during the period for income taxes		3,063	3,837
Cash paid during the period for interest		6,894	11,502
Non-cash investing and financing activities
Decrease in net loans associated with significant disposals reclassified to HFS	[1],[2]	8,291	0
Transfers to loans HFS from loans	[1],[2]	5,329	2,122
Decrease in long-term debt associated with significant disposals reclassified to HFS	[2]	521	0
Decrease in deposits associated with significant disposals reclassified to HFS	[2]	6,912	0
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		3,604	3,605
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		3,374	(5,179)
Other investing activities		0	0
Net cash used in investing activities of continuing operations		3,374	(5,179)
Cash flows from financing activities of continuing operations
Dividends paid			(4,024)
Issuance of preferred stock		2,300	1,500
Redemption of preferred stock		(3,785)	(1,500)
Treasury stock acquired		(7,448)	(2,925)
Proceeds (repayments) from issuance of long-term debt, net		4,660	16,136
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		1,074	(7,214)
Capital contributions from (to) parent		0
Other financing activities		(328)	(408)
Net cash provided by financing activities of continuing operations		(7,486)	1,565
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash, due from banks and deposits with banks		(508)	(9)
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		4,008	3,012
Cash and due from banks (including segregated cash and other deposits)		8	12
Deposits with banks, net of allowance		4,000	3,000
Cash, due from banks and deposits with banks at end of period		4,008	3,012	4,516
Supplemental disclosure of cash flow information for continuing operations
Cash paid (received) during the period for income taxes		(1,757)	(1,263)
Cash paid during the period for interest		2,307	2,507
Non-cash investing and financing activities
Decrease in net loans associated with significant disposals reclassified to HFS		0
Transfers to loans HFS from loans		0	0
Decrease in long-term debt associated with significant disposals reclassified to HFS		0
Decrease in deposits associated with significant disposals reclassified to HFS		0
Reportable legal entities | CGMHI
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by operating activities of continuing operations		30,413	(36,465)
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		(40,065)	(35,332)
Changes in investments and advances—intercompany		(9,743)	(5,532)
Other investing activities		(42)	0
Net cash used in investing activities of continuing operations		(49,850)	(40,864)
Cash flows from financing activities of continuing operations
Dividends paid		(195)	(168)
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		11,336	6,606
Proceeds (repayments) from issuance of long-term debt—intercompany, net		9,084	1,607
Change in deposits		0	0
Change in securities loaned and sold under agreements to repurchase		(2,397)	75,977
Change in short-term borrowings		2,224	788
Net change in short-term borrowings and other advances—intercompany		1,253	(6,524)
Capital contributions from (to) parent		(19)
Other financing activities		0	0
Net cash provided by financing activities of continuing operations		21,286	78,286
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash, due from banks and deposits with banks		1,849	957
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		21,961	17,398
Cash and due from banks (including segregated cash and other deposits)		6,998	5,960
Deposits with banks, net of allowance		14,963	11,438
Cash, due from banks and deposits with banks at end of period		21,961	17,398	20,112
Supplemental disclosure of cash flow information for continuing operations
Cash paid (received) during the period for income taxes		809	1,177
Cash paid during the period for interest		1,687	3,988
Non-cash investing and financing activities
Decrease in net loans associated with significant disposals reclassified to HFS		0
Transfers to loans HFS from loans		0	0
Decrease in long-term debt associated with significant disposals reclassified to HFS		0
Decrease in deposits associated with significant disposals reclassified to HFS		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		25,168	10,606
Cash flows from investing activities of continuing operations
Purchases of investments		(277,874)	(276,084)
Proceeds from sales of investments		96,203	130,237
Proceeds from maturities of investments		107,361	78,476
Change in loans		6,613	23,488
Proceeds from sales and securitizations of loans		1,134	924
Change in securities borrowed and purchased under agreements to resell		(2,919)	(2,704)
Changes in investments and advances—intercompany		6,369	10,711
Other investing activities		(2,480)	(2,205)
Net cash used in investing activities of continuing operations		(65,593)	(37,157)
Cash flows from financing activities of continuing operations
Dividends paid		195	168
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		(18,507)	(4,664)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(9,084)	(1,607)
Change in deposits		73,769	192,033
Change in securities loaned and sold under agreements to repurchase		12,056	(35,089)
Change in short-term borrowings		(2,055)	(8,398)
Net change in short-term borrowings and other advances—intercompany		(2,327)	13,738
Capital contributions from (to) parent		19
Other financing activities		0	0
Net cash provided by financing activities of continuing operations		54,066	156,181
Effect of exchange rate changes on cash and due from banks		(789)	(802)
Change in cash, due from banks and deposits with banks		12,852	128,828
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		297,839	303,285
Cash and due from banks (including segregated cash and other deposits)		21,900	19,336
Deposits with banks, net of allowance		275,939	283,949
Cash, due from banks and deposits with banks at end of period		297,839	303,285	284,987
Supplemental disclosure of cash flow information for continuing operations
Cash paid (received) during the period for income taxes		4,011	3,923
Cash paid during the period for interest		2,900	5,007
Non-cash investing and financing activities
Decrease in net loans associated with significant disposals reclassified to HFS		8,291
Transfers to loans HFS from loans		5,329	2,122
Decrease in long-term debt associated with significant disposals reclassified to HFS		521
Decrease in deposits associated with significant disposals reclassified to HFS		6,912
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
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, 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 and financing activities
Decrease in net loans associated with significant disposals reclassified to HFS		0
Transfers to loans HFS from loans		0	$ 0
Decrease in long-term debt associated with significant disposals reclassified to HFS		0
Decrease in deposits associated with significant disposals reclassified to HFS		$ 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 September 30, 2021.

[2] See Note 2 for further information on significant disposals.