CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Dec. 31, 2019	Sep. 30, 2019
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations	$ (22,254)	$ (35,358)
Cash flows from investing activities of continuing operations
Purchases of investments	(276,084)	(196,733)
Proceeds from sales of investments	130,237	96,400
Proceeds from maturities of investments	78,476	91,656
Change in loans	23,488	(11,518)
Proceeds from sales and securitizations of loans	924	2,717
Change in securities borrowed and purchased under agreements to resell	(38,036)	9,559
Changes in investments and advances—intercompany	0	0
Other investing activities	(2,205)	(4,173)
Net cash used in investing activities of continuing operations	(83,200)	(12,092)
Cash flows from financing activities of continuing operations
Dividends paid	(4,024)	(4,048)
Issuance of preferred stock	1,500	1,496
Redemption of preferred stock	(1,500)	(480)
Treasury stock acquired	(2,925)	(12,495)
Proceeds (repayments) from issuance of long-term debt, net	18,078	2,276
Proceeds (repayments) from issuance of long-term debt—intercompany, net	0	0
Change in deposits	192,033	74,599
Change in securities loaned and sold under agreements to repurchase	40,888	17,279
Change in short-term borrowings		2,884
Change in short-term borrowings	(7,610)
Net change in short-term borrowings and other advances—intercompany	0	0
Capital contributions from (to) parent		0
Other financing activities	(408)	(360)
Net cash provided by financing activities of continuing operations	236,032	81,151
Effect of exchange rate changes on cash and due from banks	(802)	(1,363)
Change in cash, due from banks and deposits with banks	129,776	32,338
Cash, due from banks and deposits with banks at beginning of period	193,919	188,105
Cash, due from banks and deposits with banks at end of period	323,695	220,443
Cash and due from banks (including segregated cash and other deposits)			$ 25,308	$ 23,967	$ 24,086
Deposits with banks, net of allowance			298,387	169,952	196,357
Cash, due from banks and deposits with banks at end of period	323,695	220,443	323,695	193,919	220,443
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes	3,837	3,735
Cash paid during the period for interest	11,502	22,343
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations	3,605	23,879
Cash flows from investing activities of continuing operations
Purchases of investments	0	0
Proceeds from sales of investments	0	4
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	(5,179)	(2,045)
Other investing activities	0	0
Net cash used in investing activities of continuing operations	(5,179)	(2,041)
Cash flows from financing activities of continuing operations
Dividends paid	(4,024)	(4,048)
Issuance of preferred stock	1,500	1,496
Redemption of preferred stock	(1,500)	(480)
Treasury stock acquired	(2,925)	(12,495)
Proceeds (repayments) from issuance of long-term debt, net	16,136	(1,122)
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
Change in short-term borrowings	0
Net change in short-term borrowings and other advances—intercompany	(7,214)	(4,834)
Capital contributions from (to) parent		0
Other financing activities	(408)	(360)
Net cash provided by financing activities of continuing operations	1,565	(21,843)
Effect of exchange rate changes on cash and due from banks	0	0
Change in cash, due from banks and deposits with banks	(9)	(5)
Cash, due from banks and deposits with banks at beginning of period	3,021	3,020
Cash, due from banks and deposits with banks at end of period	3,012	3,015
Cash and due from banks (including segregated cash and other deposits)			12		15
Deposits with banks, net of allowance			3,000		3,000
Cash, due from banks and deposits with banks at end of period	3,012	3,015	3,012	3,021	3,015
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes	(1,263)	(274)
Cash paid during the period for interest	2,507	3,107
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	(36,465)	(51,748)
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	(35,332)	8,914
Changes in investments and advances—intercompany	(5,532)	(6,204)
Other investing activities	0	(44)
Net cash used in investing activities of continuing operations	(40,864)	2,666
Cash flows from financing activities of continuing operations
Dividends paid	(168)	(155)
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	6,606	10,136
Proceeds (repayments) from issuance of long-term debt—intercompany, net	1,607	(5,683)
Change in deposits	0	0
Change in securities loaned and sold under agreements to repurchase	75,977	25,598
Change in short-term borrowings		5,855
Change in short-term borrowings	(788)
Net change in short-term borrowings and other advances—intercompany	(6,524)	15,211
Capital contributions from (to) parent		(74)
Other financing activities	0	0
Net cash provided by financing activities of continuing operations	78,286	50,888
Effect of exchange rate changes on cash and due from banks	0	0
Change in cash, due from banks and deposits with banks	957	1,806
Cash, due from banks and deposits with banks at beginning of period	16,441	15,677
Cash, due from banks and deposits with banks at end of period	17,398	17,483
Cash and due from banks (including segregated cash and other deposits)			5,960		5,501
Deposits with banks, net of allowance			11,438		11,982
Cash, due from banks and deposits with banks at end of period	17,398	17,483	17,398	16,441	17,483
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes	1,177	281
Cash paid during the period for interest	3,988	8,893
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	10,606	(7,489)
Cash flows from investing activities of continuing operations
Purchases of investments	(276,084)	(196,733)
Proceeds from sales of investments	130,237	96,396
Proceeds from maturities of investments	78,476	91,656
Change in loans	23,488	(11,518)
Proceeds from sales and securitizations of loans	924	2,717
Change in securities borrowed and purchased under agreements to resell	(2,704)	645
Changes in investments and advances—intercompany	10,711	8,249
Other investing activities	(2,205)	(4,129)
Net cash used in investing activities of continuing operations	(37,157)	(12,717)
Cash flows from financing activities of continuing operations
Dividends paid	168	155
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	(4,664)	(6,738)
Proceeds (repayments) from issuance of long-term debt—intercompany, net	(1,607)	5,683
Change in deposits	192,033	74,599
Change in securities loaned and sold under agreements to repurchase	(35,089)	(8,319)
Change in short-term borrowings		(2,971)
Change in short-term borrowings	8,398
Net change in short-term borrowings and other advances—intercompany	13,738	(10,377)
Capital contributions from (to) parent		74
Other financing activities	0	0
Net cash provided by financing activities of continuing operations	156,181	52,106
Effect of exchange rate changes on cash and due from banks	(802)	(1,363)
Change in cash, due from banks and deposits with banks	128,828	30,537
Cash, due from banks and deposits with banks at beginning of period	174,457	169,408
Cash, due from banks and deposits with banks at end of period	303,285	199,945
Cash and due from banks (including segregated cash and other deposits)			19,336		18,570
Deposits with banks, net of allowance			283,949		181,375
Cash, due from banks and deposits with banks at end of period	303,285	199,945	303,285	174,457	199,945
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes	3,923	3,728
Cash paid during the period for interest	5,007	10,343
Non-cash investing activities
Transfers to loans HFS from loans	2,122	4,400
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
Change in short-term borrowings	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	$ 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