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)	Accumulated Other Comprehensive Income (Loss) Cumulative Effect, Period of Adoption, Adjustment	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	$ 724,968	$ (1,300)			$ 2,272,103			$ (315,308)	$ (1,253,089)	$ (883)	$ (33,102)		$ 50,321	$ (417)			$ 3,584		$ 459
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	59,442								41,593				17,849
Other comprehensive income (loss), net of tax	(14,329)										(13,468)		(861)
Equity-based compensation	5,817				3,930								1,879				8
Dividends to common stockholders	(53,210)								(53,210)
Earnings distributions to limited partnership interests and other noncontrolling interests	(25,143)												(25,143)
Grant of exchangeability and redemption of limited partnership interests	38,755				26,125								12,589				41
Issuance of Class A common stock (net of costs)	4,995			$ 1,399	4,922		$ 1,221						71			$ 175	2	$ 3
Redemption of FPUs	$ (2)												(2)
Contributions of capital to and from Cantor for equity-based compensation			$ 1,305			$ 764									$ 541
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member
Other	$ 63				64								(1)
Ending Balance at Jun. 30, 2020	742,760				2,309,129			(315,308)	(1,265,589)		(46,570)		57,001				3,638		459
Beginning Balance at Mar. 31, 2020	696,324				2,293,221			(315,308)	(1,289,931)		(44,082)		48,349				3,616		459
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	39,273								27,919				11,354
Other comprehensive income (loss), net of tax	(4,120)										(2,488)		(1,632)
Equity-based compensation	3,111				2,097								1,013				1
Dividends to common stockholders	(3,577)								(3,577)
Earnings distributions to limited partnership interests and other noncontrolling interests	(6,943)												(6,943)
Grant of exchangeability and redemption of limited partnership interests	13,957				9,096								4,841				20
Issuance of Class A common stock (net of costs)	4,702			0	4,690		42						11			(42)	1	0
Redemption of FPUs	(2)												(2)
Contributions of capital to and from Cantor for equity-based compensation			87			35									52
Other	(52)				(52)								0
Ending Balance at Jun. 30, 2020	742,760				2,309,129			(315,308)	(1,265,589)		(46,570)		57,001				3,638		459
Beginning Balance at Dec. 31, 2020	808,229				2,354,492			(315,313)	(1,265,504)		(28,930)		59,290				3,735		459
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	81,966								61,260				20,706
Other comprehensive income (loss), net of tax	(2,094)										(1,675)		(419)
Equity-based compensation	6,659				4,564								2,078				17
Dividends to common stockholders	(7,626)								(7,626)
Earnings distributions to limited partnership interests and other noncontrolling interests	(11,405)												(11,405)
Grant of exchangeability and redemption of limited partnership interests	42,784			513	17,740		4,037						24,638			(3,532)	406	8
Issuance of Class A common stock (net of costs)	673				662								8				3
Redemption of FPUs	(127)												(127)
Repurchase of Class A common stock	(107,783)							(90,972)					(16,811)
Forfeiture of Class A common stock, 83,765 shares	(330)				137			(416)					(51)
Contributions of capital to and from Cantor for equity-based compensation			(26,290)			(14,143)									(12,147)
Cantor purchase of Cantor units from BGC Holdings upon redemption of FPUs, 2,768,060 units	5,145												5,145
Other	(969)				(31)								(938)
Ending Balance at Jun. 30, 2021	789,345				2,367,458			(406,701)	(1,211,870)		(30,605)		66,435				4,169		459
Beginning Balance at Mar. 31, 2021	870,422				2,365,750			(318,864)	(1,226,187)		(32,284)		77,690				3,858		459
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Consolidated net income (loss)	22,841								18,169				4,672
Other comprehensive income (loss), net of tax	1,860										1,679		181
Equity-based compensation	2,487				1,722								762				3
Dividends to common stockholders	(3,852)								(3,852)
Earnings distributions to limited partnership interests and other noncontrolling interests	(4,958)												(4,958)
Grant of exchangeability and redemption of limited partnership interests	31,948			$ 488	11,736		$ 2,820						19,910			$ (2,338)	302	$ 6
Issuance of Class A common stock (net of costs)	130				126								4				0
Redemption of FPUs	(108)												(108)
Repurchase of Class A common stock	(103,386)							(87,421)					(15,965)
Forfeiture of Class A common stock, 83,765 shares	(330)				137			(416)					(51)
Contributions of capital to and from Cantor for equity-based compensation			$ (27,171)			$ (14,746)									$ (12,425)
Other	(1,026)				(87)								(939)
Ending Balance at Jun. 30, 2021	$ 789,345				$ 2,367,458			$ (406,701)	$ (1,211,870)		$ (30,605)		$ 66,435				$ 4,169		$ 459