Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Cumulative Effect, Period of Adoption, Adjustment	Cantor	Smith Mack	Additional Paid-in Capital	Additional Paid-in Capital Cantor	Additional Paid-in Capital Smith Mack	Treasury Stock	Retained Deficit	Retained Deficit Cumulative Effect, Period of Adoption, Adjustment	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest in Subsidiaries	Noncontrolling Interest in Subsidiaries Cumulative Effect, Period of Adoption, Adjustment	Noncontrolling Interest in Subsidiaries Cantor	Noncontrolling Interest in Subsidiaries Smith Mack	Class A Common Stock Common Stock	Class A Common Stock Common Stock Smith Mack	Class B Common Stock Common Stock
Beginning Balance at Dec. 31, 2019	$ 729,106	$ (1,300)			$ 2,289,064			$ (315,308)	$ (1,264,567)	$ (883)	$ (33,102)	$ 48,976	$ (417)			$ 3,584		$ 459
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	66,158								49,091			17,067
Other comprehensive income (loss), net of tax	(6,139)										(6,303)	164
Equity-based compensation	9,816				6,631							3,176				9
Dividends to common stockholders	(56,814)								(56,814)
Earnings distributions to limited partnership interests and other noncontrolling interests	(28,285)											(28,285)
Grant of exchangeability and redemption of limited partnership interests	48,286				31,785							16,443				58
Issuance of Class A common stock (net of costs)	5,236			$ 1,377	5,155		$ 1,305					77			$ 69	4	$ 3
Redemption of FPUs	(34)											(34)
Repurchase of Class A common stock	$ (6)							(5)				(1)
Contributions of capital to and from Cantor for equity-based compensation			$ 1,442			$ 849								$ 593
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Other	$ 4,402				3,694							708
Ending Balance at Sep. 30, 2020	773,245				2,338,483			(315,313)	(1,273,173)		(39,405)	58,536				3,658		459
Beginning Balance at Jun. 30, 2020	748,953				2,329,304			(315,308)	(1,278,419)		(46,570)	55,849				3,638		459
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	8,715								8,850			(135)
Other comprehensive income (loss), net of tax	8,190										7,165	1,025
Equity-based compensation	3,999				2,701							1,297				1
Dividends to common stockholders	(3,604)								(3,604)
Earnings distributions to limited partnership interests and other noncontrolling interests	(3,142)											(3,142)
Grant of exchangeability and redemption of limited partnership interests	9,531				5,660							3,854				17
Issuance of Class A common stock (net of costs)	240			(21)	233		84					6			(106)	1	1
Redemption of FPUs	(32)											(32)
Repurchase of Class A common stock	(6)							(5)				(1)
Contributions of capital to and from Cantor for equity-based compensation			137			85								52
Other	285				416							(131)
Ending Balance at Sep. 30, 2020	773,245				2,338,483			(315,313)	(1,273,173)		(39,405)	58,536				3,658		459
Beginning Balance at Dec. 31, 2020	811,297				2,375,113			(315,313)	(1,280,828)		(28,930)	57,061				3,735		459
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	64,695								47,554			17,141
Other comprehensive income (loss), net of tax	(8,496)										(7,302)	(1,194)
Equity-based compensation	13,164				9,434							3,711				19
Dividends to common stockholders	(11,444)								(11,444)
Earnings distributions to limited partnership interests and other noncontrolling interests	(8,634)											(8,634)
Grant of exchangeability and redemption of limited partnership interests	85,685			1,160	47,794		6,315					37,400			(5,166)	491	11
Acquisition of Futures Exchange Group	(9,022)				(7,616)							(1,406)
Issuance of Class A common stock (net of costs)	1,317				1,302							11				4
Redemption of FPUs	(339)											(339)
Repurchase of Class A common stock	(234,574)							(198,002)				(36,572)
Forfeiture of Class A common stock	(553)				181			(648)				(86)
Contributions of capital to and from Cantor for equity-based compensation			(28,571)			(15,849)								(12,722)
Cantor purchase of Cantor units from BGC Holdings upon redemption of FPUs, 2,768,060 units	5,145											5,145
Other	4,335				4,424							(89)
Ending Balance at Sep. 30, 2021	685,165				2,421,098			(513,963)	(1,244,718)		(36,232)	54,261				4,260		459
Beginning Balance at Jun. 30, 2021	794,360				2,392,508			(406,701)	(1,229,498)		(30,605)	64,028				4,169		459
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	(13,941)								(11,402)			(2,539)
Other comprehensive income (loss), net of tax	(6,402)										(5,627)	(775)
Equity-based compensation	6,505				4,870							1,633				2
Dividends to common stockholders	(3,818)								(3,818)
Earnings distributions to limited partnership interests and other noncontrolling interests	2,771											2,771
Grant of exchangeability and redemption of limited partnership interests	42,901			$ 647	30,054		$ 2,278					12,762			$ (1,634)	85	$ 3
Acquisition of Futures Exchange Group	(9,022)				(7,616)							(1,406)
Issuance of Class A common stock (net of costs)	644				640							3				1
Redemption of FPUs	(212)											(212)
Repurchase of Class A common stock	(126,791)							(107,030)				(19,761)
Forfeiture of Class A common stock	(223)				44			(232)				(35)
Contributions of capital to and from Cantor for equity-based compensation			$ (2,281)			$ (1,706)								$ (575)
Other	27				26							1
Ending Balance at Sep. 30, 2021	$ 685,165				$ 2,421,098			$ (513,963)	$ (1,244,718)		$ (36,232)	$ 54,261				$ 4,260		$ 459