Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Cantor	Smith Mack	Class A Common Stock	Class A Common Stock Partnership Units	Class A Common Stock Smith Mack	Class B Common Stock	Common Stock Class A Common Stock	Common Stock Class A Common Stock Smith Mack	Common Stock Class B Common Stock	Additional Paid-in Capital	Additional Paid-in Capital Cantor	Additional Paid-in Capital Smith Mack	Treasury Stock	Retained Deficit	Accumulated Other Comprehensive Income (Loss)	Noncontrolling Interest in Subsidiaries	Noncontrolling Interest in Subsidiaries Cantor	Noncontrolling Interest in Subsidiaries Smith Mack
Beginning Balance at Dec. 31, 2020	$ 811,297							$ 3,735		$ 459	$ 2,375,113			$ (315,313)	$ (1,280,828)	$ (28,930)	$ 57,061
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							19			9,434						3,711
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							491			47,794						37,400
Issuance of Class A common stock (net of costs)	1,317		$ 1,160					4	$ 11		1,302		$ 6,315				11		$ (5,166)
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
Acquisition of Futures Exchange Group	(9,022)										(7,616)						(1,406)
Other	4,335										4,424						(89)
Ending Balance at Sep. 30, 2021	$ 685,165							4,260		459	2,421,098			(513,963)	(1,244,718)	(36,232)	54,261
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share of common stock (in dollars per share)	$ 0.03
Common stock, dividends, per share, cash paid (in dollars per share)	$ 0.03
Equity-based compensation (in shares)				1,940,975
Grant of exchangeability and redemption of limited partnership interests, issuance of shares (in shares)	49,033,008			49,033,000
Issuance of class A common stock (net of costs) (in shares)				366,043			0
Redemption of FPUs (in shares)					718,434
Repurchase of class A common stock (in shares)	41,940,000			41,939,724
Forfeiture of class A common stock (in shares)	140,000			140,188
Issuance of Class A common stock and RSUs for acquisitions (in shares)				1,130,000		1,129,630
Cantor purchase of Cantor units from BGC Holdings upon redemption of FPUs (in shares)	2,768,060
Beginning Balance at Jun. 30, 2021	$ 794,360							4,169		459	2,392,508			(406,701)	(1,229,498)	(30,605)	64,028
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							2			4,870						1,633
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							85			30,054						12,762
Issuance of Class A common stock (net of costs)	644		647					1	3		640		2,278				3		(1,634)
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)
Acquisition of Futures Exchange Group	(9,022)										(7,616)						(1,406)
Other	27										26						1
Ending Balance at Sep. 30, 2021	$ 685,165							4,260		459	2,421,098			(513,963)	(1,244,718)	(36,232)	54,261
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share of common stock (in dollars per share)	$ 0.01
Common stock, dividends, per share, cash paid (in dollars per share)	$ 0.01
Equity-based compensation (in shares)				213,459
Grant of exchangeability and redemption of limited partnership interests, issuance of shares (in shares)	8,446,093			8,446,000
Issuance of class A common stock (net of costs) (in shares)				100,128			0
Redemption of FPUs (in shares)					56,730
Repurchase of class A common stock (in shares)	24,433,000			24,432,718
Forfeiture of class A common stock (in shares)	56,000			56,423
Issuance of Class A common stock and RSUs for acquisitions (in shares)				343,000		342,162
Beginning Balance at Dec. 31, 2021	$ 663,315							4,359		459	2,451,135			(623,734)	(1,171,919)	(40,548)	43,563
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	55,269														46,496		8,773
Other comprehensive income (loss), net of tax	(14,763)															(12,535)	(2,228)
Equity-based compensation	11,964							30			9,081						2,853
Dividends to common stockholders	(11,143)														(11,143)
Earnings distributions to limited partnership interests and other noncontrolling interests	(2,950)																(2,950)
Grant of exchangeability and redemption of limited partnership interests	82,939		$ 2,710					237	$ 12		61,849		$ 2,279				20,853		$ 419
Issuance of Class A common stock (net of costs)	3,632							5			3,616						11
Redemption of FPUs	(204)																(204)
Repurchase of Class A common stock	(79,295)													(66,747)			(12,548)
Forfeiture of Class A common stock	(188)										(6)			(152)			(30)
Contributions of capital to and from Cantor for equity-based compensation		(504)										(370)						(134)
Cantor purchase of Cantor units from BGC Holdings upon redemption of FPUs, 2,768,060 units		947																947
Other	(385)										(385)						0
Ending Balance at Sep. 30, 2022	$ 711,344							4,643		459	2,527,199			(690,633)	(1,136,566)	(53,083)	59,325
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share of common stock (in dollars per share)	$ 0.03
Common stock, dividends, per share, cash paid (in dollars per share)	$ 0.03
Equity-based compensation (in shares)				3,018,337
Grant of exchangeability and redemption of limited partnership interests, issuance of shares (in shares)	23,647,165			23,654,000
Issuance of class A common stock (net of costs) (in shares)				445,245			0
Redemption of FPUs (in shares)					96,447
Repurchase of class A common stock (in shares)	21,142,000			21,142,045
Forfeiture of class A common stock (in shares)	47,000			47,807
Issuance of Class A common stock and RSUs for acquisitions (in shares)				1,186,000		1,192,907
Cantor purchase of Cantor units from BGC Holdings upon redemption of FPUs (in shares)	480,175
Beginning Balance at Jun. 30, 2022	$ 730,693							4,518		459	2,499,393			(648,500)	(1,138,628)	(46,456)	59,907
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	8,225														5,762		2,463
Other comprehensive income (loss), net of tax	(7,744)															(6,627)	(1,117)
Equity-based compensation	3,975							4			3,034				0		937
Dividends to common stockholders	(3,700)														(3,700)
Earnings distributions to limited partnership interests and other noncontrolling interests	(3,091)																(3,091)
Grant of exchangeability and redemption of limited partnership interests	32,174							120			24,026						8,028
Issuance of Class A common stock (net of costs)	154							1			149						4
Redemption of FPUs	(59)																(59)
Repurchase of Class A common stock	(49,908)													(41,994)			(7,914)
Forfeiture of Class A common stock	(171)										(5)			(139)			(27)
Contributions of capital to and from Cantor for equity-based compensation		$ 818										$ 624						$ 194
Other	(22)										(22)
Ending Balance at Sep. 30, 2022	$ 711,344							$ 4,643		$ 459	$ 2,527,199			$ (690,633)	$ (1,136,566)	$ (53,083)	$ 59,325
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Dividends declared per share of common stock (in dollars per share)	$ 0.01
Common stock, dividends, per share, cash paid (in dollars per share)	$ 0.01
Equity-based compensation (in shares)				448,014
Grant of exchangeability and redemption of limited partnership interests, issuance of shares (in shares)	11,939,546			11,940,000
Issuance of class A common stock (net of costs) (in shares)				101,786			0
Redemption of FPUs (in shares)					27,087
Repurchase of class A common stock (in shares)	12,397,000			12,396,735
Forfeiture of class A common stock (in shares)	43,000			43,388
Issuance of Class A common stock and RSUs for acquisitions (in shares)				0