CONDENSED CONSOLIDATING FINANCIAL STATEMENTS - Condensed Consolidating Statement of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations	$ (37,616)	$ 6,956
Cash flows from investing activities of continuing operations
Purchases of investments	(69,673)	(41,030)
Proceeds from sales of investments	31,436	20,688
Proceeds from maturities of investments	47,363	21,509
Change in loans	(892)	(8,717)
Proceeds from sales and securitizations of loans	2,062	1,654
Proceeds from significant disposals		0
Change in federal funds sold and resales	6,189	(25,409)
Changes in investments and advances—intercompany	0	0
Other investing activities	(442)	(819)
Net cash provided by (used in) investing activities of continuing operations	16,043	(32,124)
Cash flows from financing activities of continuing operations
Dividends paid	(1,320)	(1,095)
Redemption of preferred stock	(480)	(97)
Treasury stock acquired	(4,055)	(2,378)
Proceeds (repayments) from issuance of long-term debt, net	8,984	2,887
Proceeds (repayments) from issuance of long-term debt—intercompany, net	0	0
Change in deposits	17,186	41,397
Change in federal funds purchased and repos	12,604	15,482
Change in short-term borrowings	6,976	(8,358)
Net change in short-term borrowings and other advances—intercompany	0	0
Capital contributions from (to) parent		0
Other financing activities	(358)	(475)
Net cash provided by financing activities of continuing operations	39,537	47,363
Effect of exchange rate changes on cash and due from banks	(176)	(7)
Change in cash and due from banks and deposits with banks	17,788	22,188
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	205,893	202,704
Cash and due from banks			$ 24,448	$ 23,645	$ 21,850
Deposits with banks			181,445	164,460	180,854
Cash and due from banks and deposits with banks at end of period	188,105	202,704	205,893	188,105	202,704
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes	1,325	738
Cash paid during the period for interest	6,931	4,586
Non-cash investing activities
Transfers to loans HFS from loans	2,000	900
Transfers to OREO and other repossessed assets	36	26
Reportable legal entities | Citigroup parent company
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations	10,950	5,268
Cash flows from investing activities of continuing operations
Purchases of investments	0	(7,955)
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 federal funds sold and resales	0	0
Changes in investments and advances—intercompany	(106)	(1,463)
Other investing activities	0	(729)
Net cash provided by (used in) investing activities of continuing operations	(106)	(10,147)
Cash flows from financing activities of continuing operations
Dividends paid	(1,320)	(1,095)
Redemption of preferred stock	(480)	(97)
Treasury stock acquired	(4,055)	(2,378)
Proceeds (repayments) from issuance of long-term debt, net	5,199	699
Proceeds (repayments) from issuance of long-term debt—intercompany, net	0	0
Change in deposits	0	0
Change in federal funds purchased and repos	0	0
Change in short-term borrowings	0	0
Net change in short-term borrowings and other advances—intercompany	(9,838)	14
Capital contributions from (to) parent		0
Other financing activities	(358)	(261)
Net cash provided by financing activities of continuing operations	(10,852)	(3,118)
Effect of exchange rate changes on cash and due from banks	0	0
Change in cash and due from banks and deposits with banks	(8)	(7,997)
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	3,012	3,016
Cash and due from banks			12		16
Deposits with banks			3,000		3,000
Cash and due from banks and deposits with banks at end of period	3,020	11,013	3,012	3,020	3,016
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes	306	(266)
Cash paid during the period for interest	956	883
Non-cash investing activities
Transfers to loans HFS from loans	0	0
Transfers to OREO and other repossessed assets	0	0
Reportable legal entities | CGMHI
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities of continuing operations	(30,786)	7,046
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 federal funds sold and resales	6,748	(22,167)
Changes in investments and advances—intercompany	(6,636)	(3,603)
Other investing activities	(17)	(9)
Net cash provided by (used in) investing activities of continuing operations	95	(25,779)
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	5,576	2,004
Proceeds (repayments) from issuance of long-term debt—intercompany, net	(1,295)	(412)
Change in deposits	0	0
Change in federal funds purchased and repos	15,217	11,359
Change in short-term borrowings	2,829	(409)
Net change in short-term borrowings and other advances—intercompany	9,125	8,226
Capital contributions from (to) parent		(585)
Other financing activities	0	0
Net cash provided by financing activities of continuing operations	31,452	20,183
Effect of exchange rate changes on cash and due from banks	0	0
Change in cash and due from banks and deposits with banks	761	1,450
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	16,438	14,145
Cash and due from banks			4,916		5,648
Deposits with banks			11,522		8,497
Cash and due from banks and deposits with banks at end of period	15,677	12,695	16,438	15,677	14,145
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes	57	29
Cash paid during the period for interest	2,694	1,627
Non-cash investing activities
Transfers to loans HFS from loans	0	0
Transfers to OREO and other repossessed assets	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	(17,780)	(5,358)
Cash flows from investing activities of continuing operations
Purchases of investments	(69,673)	(33,075)
Proceeds from sales of investments	31,436	20,688
Proceeds from maturities of investments	47,363	21,509
Change in loans	(892)	(8,717)
Proceeds from sales and securitizations of loans	2,062	1,654
Proceeds from significant disposals		0
Change in federal funds sold and resales	(559)	(3,242)
Changes in investments and advances—intercompany	6,742	5,066
Other investing activities	(425)	(81)
Net cash provided by (used in) investing activities of continuing operations	16,054	3,802
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	(1,791)	184
Proceeds (repayments) from issuance of long-term debt—intercompany, net	1,295	412
Change in deposits	17,186	41,397
Change in federal funds purchased and repos	(2,613)	4,123
Change in short-term borrowings	4,147	(7,949)
Net change in short-term borrowings and other advances—intercompany	713	(8,240)
Capital contributions from (to) parent		585
Other financing activities	0	(214)
Net cash provided by financing activities of continuing operations	18,937	30,298
Effect of exchange rate changes on cash and due from banks	(176)	(7)
Change in cash and due from banks and deposits with banks	17,035	28,735
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	186,443	185,543
Cash and due from banks			19,520		16,186
Deposits with banks			166,923		169,357
Cash and due from banks and deposits with banks at end of period	169,408	156,808	186,443	169,408	185,543
Supplemental disclosure of cash flow information for continuing operations
Cash paid during the year for income taxes	962	975
Cash paid during the period for interest	3,281	2,076
Non-cash investing activities
Transfers to loans HFS from loans	2,000	900
Transfers to OREO and other repossessed assets	36	26
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 federal funds sold and resales	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 federal funds purchased and repos	0	0
Change in short-term borrowings	0	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 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	$ 0