CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		$ (19,208)	$ (37,756)
Cash flows from investing activities of continuing operations
Purchases of investments		(207,701)	(118,132)
Proceeds from sales of investments		86,191	63,595
Proceeds from maturities of investments		53,909	57,684
Change in loans		7,943	(7,803)
Proceeds from sales and securitizations of loans		826	2,249
Change in securities borrowed and purchased under agreements to resell		(31,595)	10,915
Changes in investments and advances—intercompany		0	0
Other investing activities		(1,262)	(3,210)
Net cash provided by (used in) investing activities of continuing operations		(91,689)	5,298
Cash flows from financing activities of continuing operations
Dividends paid		(2,679)	(2,650)
Issuance of preferred stock		1,500	0
Redemption of preferred stock		(1,500)	(480)
Treasury stock acquired		(2,925)	(7,518)
Proceeds (repayments) from issuance of long-term debt, net		26,174	13,421
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		163,070	32,437
Change in securities loaned and sold under agreements to repurchase		49,383	3,365
Change in short-term borrowings			10,096
Net change in short-term borrowings and other advances—intercompany		0	0
Capital contributions from parent		0
Other financing activities		(407)	(359)
Net cash provided by financing activities of continuing operations		227,723	48,312
Effect of exchange rate changes on cash and due from banks		(972)	(716)
Change in cash and due from banks and deposits with banks		115,854	15,138
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		309,773	203,243
Cash and due from banks				$ 22,889	$ 23,967	$ 24,997
Deposits with banks, net of allowance				286,884	169,952	178,246
Cash and due from banks and deposits with banks at end of period		309,773	203,243	309,773	193,919	203,243
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		2,543	2,814
Cash paid during the period for interest		8,751	14,000
Non-cash investing activities
Transfers to loans HFS from loans	[1]	1,036	3,600
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		2,857	17,500
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		(7,371)	(3,336)
Other investing activities		0	0
Net cash provided by (used in) investing activities of continuing operations		(7,371)	(3,332)
Cash flows from financing activities of continuing operations
Dividends paid		(2,679)	(2,650)
Issuance of preferred stock		1,500
Redemption of preferred stock		(1,500)	(480)
Treasury stock acquired		(2,925)	(7,518)
Proceeds (repayments) from issuance of long-term debt, net		17,353	5,418
Proceeds (repayments) from issuance of long-term debt—intercompany, net			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		(6,826)	(8,584)
Capital contributions from parent		0
Other financing activities		(407)	(359)
Net cash provided by financing activities of continuing operations		4,516	(14,173)
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		2	(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		3,023	3,015
Cash and due from banks				23		15
Deposits with banks, net of allowance				3,000		3,000
Cash and due from banks and deposits with banks at end of period		3,023	3,015	3,023	3,021	3,015
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		39	154
Cash paid during the period for interest		1,757	1,753
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		(53,782)	(39,793)
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		(29,475)	9,511
Changes in investments and advances—intercompany		(4,890)	(10,607)
Other investing activities		0	(32)
Net cash provided by (used in) investing activities of continuing operations		(34,365)	(1,128)
Cash flows from financing activities of continuing operations
Dividends paid		0	0
Issuance of preferred stock		0
Redemption of preferred stock		0	0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		8,907	10,817
Proceeds (repayments) from issuance of long-term debt—intercompany, net		6,815	(3,941)
Change in deposits		0	0
Change in securities loaned and sold under agreements to repurchase		68,650	20,903
Change in short-term borrowings		1,074	4,977
Net change in short-term borrowings and other advances—intercompany		3,035	7,088
Capital contributions from parent		0
Other financing activities		(118)	0
Net cash provided by financing activities of continuing operations		88,363	39,844
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		216	(1,077)
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		16,657	14,600
Cash and due from banks				3,728		4,479
Deposits with banks, net of allowance				12,929		10,121
Cash and due from banks and deposits with banks at end of period		16,657	14,600	16,657	16,441	14,600
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		174	119
Cash paid during the period for interest		3,006	6,577
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		31,717	(15,463)
Cash flows from investing activities of continuing operations
Purchases of investments		(207,701)	(118,132)
Proceeds from sales of investments		86,191	63,591
Proceeds from maturities of investments		53,909	57,684
Change in loans		7,943	(7,803)
Proceeds from sales and securitizations of loans		826	2,249
Change in securities borrowed and purchased under agreements to resell		(2,120)	1,404
Changes in investments and advances—intercompany		12,261	13,943
Other investing activities		(1,262)	(3,178)
Net cash provided by (used in) investing activities of continuing operations		(49,953)	9,758
Cash flows from financing activities of continuing operations
Dividends paid		0	0
Issuance of preferred stock		0
Redemption of preferred stock		0	0
Treasury stock acquired		0	0
Proceeds (repayments) from issuance of long-term debt, net		(86)	(2,814)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(6,815)	3,941
Change in deposits		163,070	32,437
Change in securities loaned and sold under agreements to repurchase		(19,267)	(17,538)
Change in short-term borrowings		(5,967)	5,119
Net change in short-term borrowings and other advances—intercompany		3,791	1,496
Capital contributions from parent		0
Other financing activities		118	0
Net cash provided by financing activities of continuing operations		134,844	22,641
Effect of exchange rate changes on cash and due from banks		(972)	(716)
Change in cash and due from banks and deposits with banks		115,636	16,220
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		290,093	185,628
Cash and due from banks				19,138		20,503
Deposits with banks, net of allowance				270,955		165,125
Cash and due from banks and deposits with banks at end of period		290,093	185,628	290,093	174,457	185,628
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		2,330	2,541
Cash paid during the period for interest		3,988	5,670
Non-cash investing activities
Transfers to loans HFS from loans		1,036	3,600
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 provided by (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
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 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, net of allowance				0		0
Cash and 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 June 30, 2020.