Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	GFI Group Inc. [Member]	Additional Paid-in Capital [Member]	Contingent Class A Common Stock [Member]	Treasury Stock [Member]	Retained Earnings (Deficit) [Member]	Accumulated Other Comprehensive Income (loss) [Member]	Noncontrolling Interest in Subsidiaries [Member]	Noncontrolling Interest in Subsidiaries [Member] GFI Group Inc. [Member]	Class A Common Stock [Member]	Class B Common Stock [Member]
Beginning Balance at Dec. 31, 2013	$ 702,747		$ 745,678	$ 12,051	$ (121,753)	$ (167,923)	$ (6,060)	$ 238,379		$ 2,027	$ 348
Consolidated net income	(3,839)					4,135		(7,974)
Other comprehensive gain, 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	(102,720)		1,011		(79,205)			(24,526)
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partner units and subsequent repurchases by BGC Holdings	(13,716)							(13,716)
Re-allocation of equity due to additional investment by founding/working partners	(110)							(110)
Issuance of stock for acquisitions	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
Purchases of Newmark noncontrolling interest	(403)		(234)					(169)
Other	(693)		(30)					(690)		27
Ending Balance at Dec. 31, 2014	581,922		817,158	47,383	(200,958)	(268,920)	4,303	180,406		2,202	348
Consolidated net income	95,826					61,773		34,053
Other comprehensive gain, net of tax	(33,991)						(30,245)	(3,746)
Equity-based compensation	3,313		2,211					1,095		7
Dividends to common stockholders	(96,087)					(96,087)
Earnings distributions to limited partnership interests and other noncontrolling interests	(61,521)							(61,521)
Grant of exchangeability and redemption of limited partnership interests	75,380		48,710					26,606		64
Issuance of Class A common stock (net of costs)	944		731					212		1
Redemption of FPUs	(451)							(451)
Repurchase of Class A common stock	(6,786)				(5,203)			(1,583)
Forfeitures of restricted Class A common stock, 218,747 shares	(1,376)		575		(1,635)			(316)
Re-allocation of equity due to additional investment by founding/working partners	(80)							(80)
Issuance of stock for acquisitions	20,980		19,092	(2,684)				4,564		8
Issuance of contingent shares and limited partnership interests in connection with acquisitions	14,082			10,785				3,297
Conversion of 8.75% Convertible Notes to Class A common stock, 24,042,599 shares	150,000		117,178					32,582		240
Reclassification of Redeemable noncontrolling interest to noncontrolling interest for GFI Back-End Merger		$ 231,320							$ 231,320
Purchases of Newmark noncontrolling interest	(488)		731					(1,219)
Other	1,807							1,807
Ending Balance at Sep. 30, 2015	$ 974,794		$ 1,006,386	$ 55,484	$ (207,796)	$ (303,234)	$ (25,942)	$ 447,026		$ 2,522	$ 348