CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		$ 23,198	$ (25,533)
Cash flows from investing activities of continuing operations
Purchases of investments		(111,187)	(108,658)
Proceeds from sales of investments		46,049	44,399
Proceeds from maturities of investments		35,088	29,203
Change in loans		9,933	(26,743)
Proceeds from sales and securitizations of loans		323	596
Change in securities borrowed and purchased under agreements to resell		(20,360)	(11,214)
Changes in investments and advances—intercompany		0	0
Other investing activities		(780)	(440)
Net cash used in investing activities of continuing operations		(40,934)	(72,857)
Cash flows from financing activities of continuing operations
Dividends paid		(1,356)	(1,365)
Issuance of preferred stock		2,300	1,500
Redemption of preferred stock		(1,500)	(1,500)
Treasury stock acquired		(1,481)	(2,925)
Proceeds (repayments) from issuance of long-term debt, net		(6,916)	15,846
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		20,304	114,321
Change in securities loaned and sold under agreements to repurchase		19,643	55,985
Change in short-term borrowings		2,573	9,902
Net change in short-term borrowings and other advances—intercompany		0	0
Other financing activities		(312)	(406)
Net cash provided by financing activities of continuing operations		33,255	191,358
Effect of exchange rate changes on cash and due from banks		(452)	(967)
Change in cash, due from banks and deposits with banks		15,067	92,001
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		324,682	285,920
Cash and due from banks (including segregated cash and other deposits)				$ 26,204	$ 26,349	$ 23,755
Deposits with banks, net of allowance				298,478	283,266	262,165
Cash, due from banks and deposits with banks at end of period		324,682	285,920	324,682	309,615	285,920
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		950	1,441
Cash paid during the period for interest		1,729	5,424
Non-cash investing activities
Transfers to loans HFS from loans	[1]	636	224
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		(4,966)	4,334
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		1,887	1,121
Other investing activities		0	0
Net cash used in investing activities of continuing operations		1,887	1,121
Cash flows from financing activities of continuing operations
Dividends paid		(1,356)	(1,365)
Issuance of preferred stock		2,300	1,500
Redemption of preferred stock		(1,500)	(1,500)
Treasury stock acquired		(1,481)	(2,925)
Proceeds (repayments) from issuance of long-term debt, net		(1,039)	5,742
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		4,962	(6,507)
Other financing activities		(312)	(406)
Net cash provided by financing activities of continuing operations		1,574	(5,461)
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash, due from banks and deposits with banks		(1,505)	(6)
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		3,011	3,015
Cash and due from banks (including segregated cash and other deposits)				11		15
Deposits with banks, net of allowance				3,000		3,000
Cash, due from banks and deposits with banks at end of period		3,011	3,015	3,011	4,516	3,015
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		99	16
Cash paid during the period for interest		126	998
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 (used in) operating activities of continuing operations		12,638	(38,869)
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		(21,547)	(8,421)
Changes in investments and advances—intercompany		(2,991)	(9,442)
Other investing activities		(23)	0
Net cash used in investing activities of continuing operations		(24,561)	(17,863)
Cash flows from financing activities of continuing operations
Dividends paid		(115)	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		3,172	72
Proceeds (repayments) from issuance of long-term debt—intercompany, net		5,702	554
Change in deposits		0	0
Change in securities loaned and sold under agreements to repurchase		3,752	49,341
Change in short-term borrowings		551	2,901
Net change in short-term borrowings and other advances—intercompany		(405)	7,040
Other financing activities		0	(119)
Net cash provided by financing activities of continuing operations		12,657	59,789
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash, due from banks and deposits with banks		734	3,057
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		20,846	19,498
Cash and due from banks (including segregated cash and other deposits)				6,605		4,525
Deposits with banks, net of allowance				14,241		14,973
Cash, due from banks and deposits with banks at end of period		20,846	19,498	20,846	20,112	19,498
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		31	78
Cash paid during the period for interest		634	1,983
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 (used in) operating activities of continuing operations		15,526	9,002
Cash flows from investing activities of continuing operations
Purchases of investments		(111,187)	(108,658)
Proceeds from sales of investments		46,049	44,399
Proceeds from maturities of investments		35,088	29,203
Change in loans		9,933	(26,743)
Proceeds from sales and securitizations of loans		323	596
Change in securities borrowed and purchased under agreements to resell		1,187	(2,793)
Changes in investments and advances—intercompany		1,104	8,321
Other investing activities		(757)	(440)
Net cash used in investing activities of continuing operations		(18,260)	(56,115)
Cash flows from financing activities of continuing operations
Dividends paid		115	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		(9,049)	10,032
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(5,702)	(554)
Change in deposits		20,304	114,321
Change in securities loaned and sold under agreements to repurchase		15,891	6,644
Change in short-term borrowings		2,022	7,001
Net change in short-term borrowings and other advances—intercompany		(4,557)	(533)
Other financing activities		0	119
Net cash provided by financing activities of continuing operations		19,024	137,030
Effect of exchange rate changes on cash and due from banks		(452)	(967)
Change in cash, due from banks and deposits with banks		15,838	88,950
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		300,825	263,407
Cash and due from banks (including segregated cash and other deposits)				19,588		19,215
Deposits with banks, net of allowance				281,237		244,192
Cash, due from banks and deposits with banks at end of period		300,825	263,407	300,825	284,987	263,407
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		820	1,347
Cash paid during the period for interest		969	2,443
Non-cash investing activities
Transfers to loans HFS from loans		636	224
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
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	$ 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

[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 March 31, 2021.