Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Cantor [Member]	Smith Mack [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Cantor [Member]	Additional Paid-in Capital [Member] Smith Mack [Member]	Treasury Stock [Member]	Retained Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest in Subsidiaries [Member]	Noncontrolling Interest in Subsidiaries [Member] Cantor [Member]	Noncontrolling Interest in Subsidiaries [Member] Smith Mack [Member]	Class A Common Stock [Member] Common Stock [Member]	Class A Common Stock [Member] Common Stock [Member] Smith Mack [Member]	Class B Common Stock [Member] Common Stock [Member]
Beginning Balance at Dec. 31, 2019	$ 724,968			$ 2,272,103			$ (315,308)	$ (1,253,089)	$ (33,102)	$ 50,321			$ 3,584		$ 459
Consolidated net income (loss)	20,169							13,674		6,495
Other comprehensive income (loss), net of tax	(10,209)								(10,980)	771
Equity-based compensation	2,706			1,833						866			7
Dividends to common stockholders	(49,633)							(49,633)
Earnings distributions to limited partnership interests and other noncontrolling interests	(18,200)									(18,200)
Grant of exchangeability and redemption of limited partnership interests	24,798			17,029						7,748			21
Issuance of Class A common stock (net of costs)	293		$ 1,399	232		$ 1,179				60		$ 217	1	$ 3
Contributions of capital to and from Cantor for equity-based compensation		$ 1,218			$ 729						$ 489
Cumulative effect of CECL standard adoption	(1,300)							(883)		(417)
Other	115			116						(1)
Ending Balance at Mar. 31, 2020	696,324			2,293,221			(315,308)	(1,289,931)	(44,082)	48,349			3,616		459
Beginning Balance at Dec. 31, 2020	808,229			2,354,492			(315,313)	(1,265,504)	(28,930)	59,290			3,735		459
Consolidated net income (loss)	59,125							43,091		16,034
Other comprehensive income (loss), net of tax	(3,954)								(3,354)	(600)
Equity-based compensation	4,172			2,842						1,316			14
Dividends to common stockholders	(3,774)							(3,774)
Earnings distributions to limited partnership interests and other noncontrolling interests	(6,447)									(6,447)
Grant of exchangeability and redemption of limited partnership interests	10,836		$ 25	6,004		$ 1,217				4,728		$ (1,194)	104	$ 2
Issuance of Class A common stock (net of costs)	543			536						4			3
Redemption of FPUs	(19)									(19)
Repurchase of Class A common stock	(4,397)						(3,551)			(846)
Contributions of capital to and from Cantor for equity-based compensation		$ 881			$ 603						$ 278
Cantor purchase of Cantor units from BGC Holdings upon redemption of FPUs, 2,768,060 units	5,145									5,145
Other	57			56						1
Ending Balance at Mar. 31, 2021	$ 870,422			$ 2,365,750			$ (318,864)	$ (1,226,187)	$ (32,284)	$ 77,690			$ 3,858		$ 459