Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Smith Mack [Member]	Lucera [Member]	GFI Group Inc. [Member]	Newmark [Member]	ELX [Member]	4.50% Convertible Notes [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Smith Mack [Member]	Additional Paid-in Capital [Member] GFI Group Inc. [Member]	Additional Paid-in Capital [Member] Newmark [Member]	Additional Paid-in Capital [Member] ELX [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] Smith Mack [Member]	Noncontrolling Interest in Subsidiaries [Member] Lucera [Member]	Noncontrolling Interest in Subsidiaries [Member] GFI Group Inc. [Member]	Noncontrolling Interest in Subsidiaries [Member] Newmark [Member]	Noncontrolling Interest in Subsidiaries [Member] ELX [Member]	Noncontrolling Interest in Subsidiaries [Member] 4.50% Convertible Notes [Member]	Class A Common Stock [Member]	Class A Common Stock [Member] Smith Mack [Member]	Class A Common Stock [Member] GFI Group Inc. [Member]	Class B Common Stock [Member]
Beginning 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, 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 partners units	2,357																		2,357
Issuance of Class A common stock for acquisitions/Mergers		$ 8,128		$ (16)					$ 15,683	$ 258,440					$ (9,301)					$ 1,728		$ (258,691)					$ 18	$ 235
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
Consolidated net income (loss)	51,045																39,983		11,062
Other comprehensive gain, net of tax	11,743																	10,198	1,545
Equity-based compensation	2,582							1,648											930							4
Dividends to common stockholders	(96,510)																(96,510)
Earnings distributions to limited partnership interests and other noncontrolling interests	(65,815)																		(65,815)
Grant of exchangeability and redemption of limited partnership interests	71,803							44,654											27,095							54
Issuance of Class A common stock (net of costs)	394							309											85
Redemption of FPUs	(2,284)																		(2,284)
Repurchase of Class A common stock	(10,590)															(8,319)			(2,271)
Forfeitures of restricted Class A common stock	(224)							140								(316)			(48)
Issuance of Class A common stock for acquisitions/Mergers	2,662							6,237						(4,156)					567							14
Issuance of contingent shares and limited partnership interests in connection with acquisitions	1,563							1,229											334
Purchase of noncontrolling interest					$ (1,315)						$ (175)												$ (1,140)
Other	638							(1)											639
Ending Balance at Jun. 30, 2017	$ 1,119,430							$ 1,520,627						$ 38,316		$ (297,378)	$ (415,053)	$ (13,001)	$ 282,574							$ 2,997			$ 348