CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations	$ (37,756)	$ 8,319
Cash flows from investing activities of continuing operations
Purchases of investments	(118,132)	(81,871)
Proceeds from sales of investments	63,595	41,808
Proceeds from maturities of investments	57,684	44,846
Change in loans	(7,803)	(10,132)
Proceeds from sales and securitizations of loans	2,249	3,217
Change in securities borrowed and purchased under agreements to resell	10,915	(33,048)
Changes in investments and advances—intercompany	0	0
Other investing activities	(3,210)	(1,449)
Net cash provided by (used in) investing activities of continuing operations	5,298	(36,629)
Cash flows from financing activities of continuing operations
Dividends paid	(2,650)	(2,232)
Redemption of preferred stock	(480)	(218)
Treasury stock acquired	(7,518)	(4,686)
Proceeds (repayments) from issuance of long-term debt, net	13,421	5,670
Proceeds (repayments) from issuance of long-term debt—intercompany, net	0	0
Change in deposits	32,437	36,908
Change in securities loaned and sold under agreements to repurchase	3,365	21,551
Change in short-term borrowings	10,096	(7,219)
Net change in short-term borrowings and other advances—intercompany	0	0
Capital contributions from parent		0
Other financing activities	(359)	(475)
Net cash provided by financing activities of continuing operations	48,312	49,299
Effect of exchange rate changes on cash and due from banks	(716)	(603)
Change in cash and due from banks and deposits with banks	15,138	20,386
Cash, due from banks and deposits with banks at beginning of period	188,105	180,516
Cash, due from banks and deposits with banks at end of period	203,243	200,902
Cash and due from banks			$ 24,997	$ 23,645	$ 21,077
Deposits with banks			178,246	164,460	179,825
Cash and due from banks and deposits with banks at end of period	188,105	180,516	203,243	188,105	200,902
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes	2,814	2,239
Cash paid during the period for interest	14,000	9,957
Non-cash investing activities
Transfers to loans HFS from loans	3,600	2,900
Transfers to OREO and other repossessed assets		55
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations	17,500	5,156
Cash flows from investing activities of continuing operations
Purchases of investments	0	(7,955)
Proceeds from sales of investments	4	7,634
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	(3,336)	(4,780)
Other investing activities	0	212
Net cash provided by (used in) investing activities of continuing operations	(3,332)	(4,889)
Cash flows from financing activities of continuing operations
Dividends paid	(2,650)	(2,232)
Redemption of preferred stock	(480)	(218)
Treasury stock acquired	(7,518)	(4,686)
Proceeds (repayments) from issuance of long-term debt, net	5,418	(1,167)
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	32
Net change in short-term borrowings and other advances—intercompany	(8,584)	497
Capital contributions from parent		0
Other financing activities	(359)	(475)
Net cash provided by financing activities of continuing operations	(14,173)	(8,249)
Effect of exchange rate changes on cash and due from banks	0	0
Change in cash and due from banks and deposits with banks	(5)	(7,982)
Cash, due from banks and deposits with banks at beginning of period	3,020	11,013
Cash, due from banks and deposits with banks at end of period	3,015	3,031
Cash and due from banks			15		31
Deposits with banks			3,000		3,000
Cash and due from banks and deposits with banks at end of period	3,020	11,013	3,015	3,020	3,031
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes	154	941
Cash paid during the period for interest	1,753	1,729
Non-cash investing activities
Transfers to loans HFS from loans	0	0
Transfers to OREO and other repossessed assets		0
Reportable legal entities | CGMHI
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations	(39,793)	1,207
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	9,511	(30,331)
Changes in investments and advances—intercompany	(10,607)	(1,872)
Other investing activities	(32)	(26)
Net cash provided by (used in) investing activities of continuing operations	(1,128)	(32,229)
Cash flows from financing activities of continuing operations
Dividends paid	0	0
Redemption of preferred stock	0	0
Treasury stock acquired	0	0
Proceeds (repayments) from issuance of long-term debt, net	10,817	5,805
Proceeds (repayments) from issuance of long-term debt—intercompany, net	(3,941)	(1,025)
Change in deposits	0	0
Change in securities loaned and sold under agreements to repurchase	20,903	26,367
Change in short-term borrowings	4,977	(459)
Net change in short-term borrowings and other advances—intercompany	7,088	1,704
Capital contributions from parent		(663)
Other financing activities	0	0
Net cash provided by financing activities of continuing operations	39,844	31,729
Effect of exchange rate changes on cash and due from banks	0	0
Change in cash and due from banks and deposits with banks	(1,077)	707
Cash, due from banks and deposits with banks at beginning of period	15,677	12,695
Cash, due from banks and deposits with banks at end of period	14,600	13,402
Cash and due from banks			4,479		4,242
Deposits with banks			10,121		9,160
Cash and due from banks and deposits with banks at end of period	15,677	12,695	14,600	15,677	13,402
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes	119	42
Cash paid during the period for interest	6,577	3,676
Non-cash investing activities
Transfers to loans HFS from loans	0	0
Transfers to OREO and other repossessed assets		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	(15,463)	1,956
Cash flows from investing activities of continuing operations
Purchases of investments	(118,132)	(73,916)
Proceeds from sales of investments	63,591	34,174
Proceeds from maturities of investments	57,684	44,846
Change in loans	(7,803)	(10,132)
Proceeds from sales and securitizations of loans	2,249	3,217
Change in securities borrowed and purchased under agreements to resell	1,404	(2,717)
Changes in investments and advances—intercompany	13,943	6,652
Other investing activities	(3,178)	(1,635)
Net cash provided by (used in) investing activities of continuing operations	9,758	489
Cash flows from financing activities of continuing operations
Dividends paid	0	0
Redemption of preferred stock	0	0
Treasury stock acquired	0	0
Proceeds (repayments) from issuance of long-term debt, net	(2,814)	1,032
Proceeds (repayments) from issuance of long-term debt—intercompany, net	3,941	1,025
Change in deposits	32,437	36,908
Change in securities loaned and sold under agreements to repurchase	(17,538)	(4,816)
Change in short-term borrowings	5,119	(6,792)
Net change in short-term borrowings and other advances—intercompany	1,496	(2,201)
Capital contributions from parent		663
Other financing activities	0	0
Net cash provided by financing activities of continuing operations	22,641	25,819
Effect of exchange rate changes on cash and due from banks	(716)	(603)
Change in cash and due from banks and deposits with banks	16,220	27,661
Cash, due from banks and deposits with banks at beginning of period	169,408	156,808
Cash, due from banks and deposits with banks at end of period	185,628	184,469
Cash and due from banks			20,503		16,804
Deposits with banks			165,125		167,665
Cash and due from banks and deposits with banks at end of period	169,408	156,808	185,628	169,408	184,469
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes	2,541	1,256
Cash paid during the period for interest	5,670	4,552
Non-cash investing activities
Transfers to loans HFS from loans	3,600	2,900
Transfers to OREO and other repossessed assets		55
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
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			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 year 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
Transfers to OREO and other repossessed assets		$ 0