CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		$ (25,533)	$ (37,616)
Cash flows from investing activities of continuing operations
Purchases of investments		(108,658)	(69,673)
Proceeds from sales of investments		44,399	31,436
Proceeds from maturities of investments		29,203	47,363
Change in loans		(26,743)	(892)
Proceeds from sales and securitizations of loans		596	2,062
Proceeds from significant disposals	[1]		0
Change in securities borrowed and purchased under agreements to resell		(11,214)	6,189
Changes in investments and advances—intercompany		0	0
Other investing activities		(440)	(442)
Net cash provided by (used in) investing activities of continuing operations		(72,857)	16,043
Cash flows from financing activities of continuing operations
Dividends paid		(1,365)	(1,320)
Issuance of preferred stock		1,500	0
Redemption of preferred stock		(1,500)	(480)
Treasury stock acquired		(2,925)	(4,055)
Proceeds (repayments) from issuance of long-term debt, net		15,846	8,984
Proceeds (repayments) from issuance of long-term debt—intercompany, net		0	0
Change in deposits		114,321	17,186
Change in securities loaned and sold under agreements to repurchase		55,985	12,604
Change in short-term borrowings		9,902	6,976
Net change in short-term borrowings and other advances—intercompany		0	0
Capital contributions from parent		0
Other financing activities		(406)	(358)
Net cash provided by (used in) financing activities of continuing operations		191,358	39,537
Effect of exchange rate changes on cash and due from banks		(967)	(176)
Change in cash and due from banks and deposits with banks		92,001	17,788
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		285,920	205,893
Cash and due from banks				$ 23,755	$ 23,967	$ 24,448
Deposits with banks, net of allowance				262,165	169,952	181,445
Cash and due from banks and deposits with banks at end of period		285,920	188,105	285,920	193,919	205,893
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		1,441	1,325
Cash paid during the period for interest		5,424	6,931
Non-cash investing activities
Transfers to loans HFS from loans	[1]	224	2,000
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations		4,334	10,950
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
Proceeds from significant disposals			0
Change in securities borrowed and purchased under agreements to resell		0	0
Changes in investments and advances—intercompany		1,121	(106)
Other investing activities		0	0
Net cash provided by (used in) investing activities of continuing operations		1,121	(106)
Cash flows from financing activities of continuing operations
Dividends paid		(1,365)	(1,320)
Issuance of preferred stock		1,500
Redemption of preferred stock		(1,500)	(480)
Treasury stock acquired		(2,925)	(4,055)
Proceeds (repayments) from issuance of long-term debt, net		5,742	5,199
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		(6,507)	(9,838)
Capital contributions from parent		0
Other financing activities		(406)	(358)
Net cash provided by (used in) financing activities of continuing operations		(5,461)	(10,852)
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		(6)	(8)
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		3,015	3,012
Cash and due from banks				15		12
Deposits with banks, net of allowance				3,000		3,000
Cash and due from banks and deposits with banks at end of period		3,015	3,012	3,015	3,021	3,012
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		16	306
Cash paid during the period for interest		998	956
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		(38,869)	(30,786)
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
Proceeds from significant disposals			0
Change in securities borrowed and purchased under agreements to resell		(8,421)	6,748
Changes in investments and advances—intercompany		(9,442)	(6,636)
Other investing activities		0	(17)
Net cash provided by (used in) investing activities of continuing operations		(17,863)	95
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		72	5,576
Proceeds (repayments) from issuance of long-term debt—intercompany, net		554	(1,295)
Change in deposits		0	0
Change in securities loaned and sold under agreements to repurchase		49,341	15,217
Change in short-term borrowings		2,901	2,829
Net change in short-term borrowings and other advances—intercompany		7,040	9,125
Capital contributions from parent		0
Other financing activities		(119)	0
Net cash provided by (used in) financing activities of continuing operations		59,789	31,452
Effect of exchange rate changes on cash and due from banks		0	0
Change in cash and due from banks and deposits with banks		3,057	761
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		19,498	16,438
Cash and due from banks				4,525		4,916
Deposits with banks, net of allowance				14,973		11,522
Cash and due from banks and deposits with banks at end of period		19,498	16,438	19,498	16,441	16,438
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		78	57
Cash paid during the period for interest		1,983	2,694
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		9,002	(17,780)
Cash flows from investing activities of continuing operations
Purchases of investments		(108,658)	(69,673)
Proceeds from sales of investments		44,399	31,436
Proceeds from maturities of investments		29,203	47,363
Change in loans		(26,743)	(892)
Proceeds from sales and securitizations of loans		596	2,062
Proceeds from significant disposals			0
Change in securities borrowed and purchased under agreements to resell		(2,793)	(559)
Changes in investments and advances—intercompany		8,321	6,742
Other investing activities		(440)	(425)
Net cash provided by (used in) investing activities of continuing operations		(56,115)	16,054
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		10,032	(1,791)
Proceeds (repayments) from issuance of long-term debt—intercompany, net		(554)	1,295
Change in deposits		114,321	17,186
Change in securities loaned and sold under agreements to repurchase		6,644	(2,613)
Change in short-term borrowings		7,001	4,147
Net change in short-term borrowings and other advances—intercompany		(533)	713
Capital contributions from parent		0
Other financing activities		119	0
Net cash provided by (used in) financing activities of continuing operations		137,030	18,937
Effect of exchange rate changes on cash and due from banks		(967)	(176)
Change in cash and due from banks and deposits with banks		88,950	17,035
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		263,407	186,443
Cash and due from banks				19,215		19,520
Deposits with banks, net of allowance				244,192		166,923
Cash and due from banks and deposits with banks at end of period		263,407	186,443	263,407	174,457	186,443
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the period for income taxes		1,347	962
Cash paid during the period for interest		2,443	3,281
Non-cash investing activities
Transfers to loans HFS from loans		224	2,000
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
Proceeds from significant disposals			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 (used in) 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 March 31, 2020.