Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	GFI Group Inc. [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Smith Mack [Member]	Additional Paid-in Capital [Member] GFI Group Inc. [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]	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, 2014	$ 581,922		$ 817,158			$ 47,383		$ (200,958)	$ (268,920)	$ 4,303	$ 180,406		$ 2,202			$ 348
Consolidated net income	268,318								126,788		141,530
Other comprehensive gain, 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
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		$ 222,148										$ 222,148
Purchases of Newmark noncontrolling interest	(488)		731								(1,219)
Other	1,806		(2)								1,806		2
Ending Balance at Dec. 31, 2015	1,242,570		1,109,000			50,095		(212,331)	(273,492)	(25,056)	591,447		2,559			348
Consolidated net income	16,291								13,659		2,632
Other comprehensive gain, net of tax	6,221									5,983	238
Equity-based compensation	1,493		987								502		4
Dividends to common stockholders	(37,980)								(37,980)
Earnings distributions to limited partnership interests and other noncontrolling interests	(15,697)										(15,697)
Grant of exchangeability and redemption of limited partnership interests	18,956		11,730								7,217		9
Issuance of Class A common stock (net of costs)	175		138								37
Redemption of FPUs	(508)										(508)
Repurchase of Class A common stock	(62,679)							(49,475)			(13,204)
Forfeitures of restricted Class A common stock	(23)		8					(26)			(5)
Issuance of Class A common stock for acquisitions/Mergers		$ (15)		$ 337	$ 258,440		$ (338)					$ (258,690)		$ 1	$ 235
Issuance of contingent shares and limited partnership interests in connection with acquisitions	666		369			157					140
Other	(3)		(655)			1			(3)		655		(1)
Ending Balance at Mar. 31, 2016	$ 1,169,467		$ 1,380,354			$ 49,915		$ (261,832)	$ (297,816)	$ (19,073)	$ 314,764		$ 2,807			$ 348