Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	GFI Group Inc. [Member]	Smith Mack [Member]	Lucera [Member]	Newmark [Member]	ELX [Member]	8.75% Convertible Notes [Member]	4.50% Convertible Notes [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] GFI Group Inc. [Member]	Additional Paid-in Capital [Member] Smith Mack [Member]	Additional Paid-in Capital [Member] Newmark [Member]	Additional Paid-in Capital [Member] ELX [Member]	Additional Paid-in Capital [Member] 8.75% Convertible Notes [Member]	Additional Paid-in Capital [Member] 4.50% Convertible Notes [Member]	Contingent Class A Common Stock [Member]	Contingent Class A Common Stock [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] GFI Group Inc. [Member]	Noncontrolling Interest in Subsidiaries [Member] Smith Mack [Member]	Noncontrolling Interest in Subsidiaries [Member] Lucera [Member]	Noncontrolling Interest in Subsidiaries [Member] Newmark [Member]	Noncontrolling Interest in Subsidiaries [Member] ELX [Member]	Noncontrolling Interest in Subsidiaries [Member] 8.75% Convertible Notes [Member]	Noncontrolling Interest in Subsidiaries [Member] 4.50% Convertible Notes [Member]	Class A Common Stock [Member]	Class A Common Stock [Member] GFI Group Inc. [Member]	Class A Common Stock [Member] Smith Mack [Member]	Class A Common Stock [Member] 8.75% Convertible Notes [Member]	Class B Common Stock [Member]
Beginning Balance at Dec. 31, 2013	$ 698,730								$ 745,678							$ 12,051		$ (121,753)	$ (171,984)	$ (6,060)	$ 238,423								$ 2,027				$ 348
Consolidated net income (loss)	(14,413)																		(3,050)		(11,363)
Other comprehensive gain (loss), net of tax	12,301																			10,363	1,938
Equity-based compensation	3,332								2,275												1,047								10
Dividends to common stockholders	(105,132)																		(105,132)
Earnings distributions to limited partnership interests and other noncontrolling interests	(55,821)																				(55,821)
Grant of exchangeability and redemption of limited partnership interests	90,067								59,207												30,741								119
Issuance of Class A common stock (net of costs)	361								275												86
Redemption of FPUs	(2,359)																				(2,359)
Repurchase of Class A common stock	(100,268)								5									(76,322)			(23,951)
Forfeitures of restricted Class A common stock	(2,452)								1,006									(2,883)			(575)
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partner units	(13,716)																				(13,716)
Re-allocation of equity due to additional investment by founding/working partners	(110)																				(110)
Issuance of Class A common stock for acquisitions/Mergers	7,000								8,976							(3,635)					1,640								19
Issuance of contingent shares and limited partnership interests in connection with acquisitions	50,907															38,967					11,940
Deemed contribution due to acquisition of Lucera				$ 12,158																				$ 12,158
Purchase of noncontrolling interest					$ (403)							$ (234)													$ (169)
Other	(693)								(30)												(690)								27
Ending Balance at Dec. 31, 2014	579,489								817,158							47,383		(200,958)	(280,166)	4,303	189,219								2,202				348
Consolidated net income (loss)	260,115																		121,318		138,797
Other comprehensive gain (loss), net of tax	(33,235)																			(29,359)	(3,876)
Equity-based compensation	4,371								2,909												1,454								8
Dividends to common stockholders	(131,360)																		(131,360)
Earnings distributions to limited partnership interests and other noncontrolling interests	(70,538)																				(70,538)
Grant of exchangeability and redemption of limited partnership interests	217,040								141,262												75,684								94
Issuance of Class A common stock (net of costs)	1,108								860												247								1
Redemption of FPUs	(835)																				(835)
Repurchase of Class A common stock	(12,114)																	(9,371)			(2,743)
Forfeitures of restricted Class A common stock	(1,701)								688									(2,002)			(387)
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partner units	6,573																				6,573
Re-allocation of equity due to additional investment by founding/working partners	(80)																				(80)
Issuance of Class A common stock for acquisitions/Mergers	545								5,112							(4,579)													12
Issuance of contingent shares and limited partnership interests in connection with acquisitions	39,090								23,104							7,291					8,695
Reclassification of Redeemable noncontrolling interest to noncontrolling interest for GFI Back-End Merger		$ 222,148																				$ 222,148
Conversion of Convertible Notes to Class A common stock							$ 150,000							$ 117,178													$ 32,582					$ 240
Purchase of noncontrolling interest					(488)							731													(1,219)
Other	1,806								(2)												1,806								2
Ending Balance at Dec. 31, 2015	1,231,934								1,109,000							50,095		(212,331)	(290,208)	(25,056)	597,527								2,559				348
Consolidated net income (loss)	128,008																		102,477		25,531
Other comprehensive gain (loss), net of tax	1,124																			1,857	(733)
Equity-based compensation	4,568								2,999												1,563								6
Dividends to common stockholders	(170,795)																		(170,795)
Earnings distributions to limited partnership interests and other noncontrolling interests	(62,258)																				(62,258)
Grant of exchangeability and redemption of limited partnership interests	95,682								58,751												36,844								87
Issuance of Class A common stock (net of costs)	17,778								14,216												3,542								20
Redemption of FPUs	(2,307)																				(2,307)
Repurchase of Class A common stock	(96,178)																	(75,984)			(20,194)
Forfeitures of restricted Class A common stock	(374)								132									(428)			(78)
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partner units	2,357																				2,357
Issuance of Class A common stock for acquisitions/Mergers		$ (16)	$ 8,128							$ 258,440	$ 15,683						$ (9,301)					$ (258,691)	$ 1,728							$ 235	$ 18
Issuance of contingent shares and limited partnership interests in connection with acquisitions	12,802								8,425							1,678					2,699
Acquisition of Lucera	(29,037)																				(29,037)
Deemed contribution due to acquisition of Lucera				$ 15,005																				$ 15,005
Conversion of Convertible Notes to Class A common stock								$ 68							$ 54													$ 14
Purchase of noncontrolling interest					$ (2,059)	$ (14)						$ (451)	$ (11)												$ (1,608)	$ (3)
Other	(678)								(652)												(26)
Ending Balance at Dec. 31, 2016	$ 1,153,738								$ 1,466,586							$ 42,472		$ (288,743)	$ (358,526)	$ (23,199)	$ 311,875								$ 2,925				$ 348