Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Commercial Real Estate Consulting Firm [Member]	Smith Mack [Member]	Berkeley Point [Member]	Newmark [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Smith Mack [Member]	Additional Paid-in Capital [Member] Berkeley Point [Member]	Additional Paid-in Capital [Member] Newmark [Member]	Contingent Class A Common Stock [Member]	Contingent Class A Common Stock [Member] Smith Mack [Member]	Treasury Stock [Member]	Retained Deficit [Member]	Retained Deficit [Member] Berkeley Point [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest in Subsidiaries [Member]	Noncontrolling Interest in Subsidiaries [Member] Commercial Real Estate Consulting Firm [Member]	Noncontrolling Interest in Subsidiaries [Member] Smith Mack [Member]	Noncontrolling Interest in Subsidiaries [Member] Berkeley Point [Member]	Noncontrolling Interest in Subsidiaries [Member] Newmark [Member]	Class A Common Stock [Member]	Class A Common Stock [Member] Smith Mack [Member]	Class B Common Stock [Member]
Beginning Balance at Dec. 31, 2016	$ 1,633,540					$ 1,662,615				$ 42,472		$ (288,743)	$ (212,858)		$ (23,199)	$ 449,980					$ 2,925		$ 348
Consolidated net income (loss)	81,729												51,475			30,254
Other comprehensive gain, net of tax	14,852														12,713	2,139
Equity-based compensation	2,137					1,370										761					6
Dividends to common stockholders	(200,116)												(200,116)
Earnings distributions to limited partnership interests and other noncontrolling interests	(118,795)															(118,795)
Grant of exchangeability and redemption of limited partnership interests	203,199					125,502										77,605					92
Issuance of Class A common stock (net of costs)	57,721					45,101										12,600					20
Redemption of FPUs	(6,665)															(6,665)
Repurchase of Class A common stock	(16,773)											(13,164)				(3,609)
Forfeitures of restricted Class A common stock	(1,153)					1,063						(1,966)				(250)
Cantor purchase of Cantor units from BGC Holdings upon redemption of founding/working partners units	3,920															3,920
Issuance of Class A common stock for acquisitions			$ 2,662				$ 8,037				$ (5,961)							$ 566				$ 20
Issuance of contingent shares and limited partnership interests in connection with acquisitions	11,570					5,121				3,961						2,488
Acquisition	(89,149)												(56,483)			(32,666)
Noncontrolling Interest Decrease From Business Combination				$ (766,406)				$ (87,434)						$ (441,386)					$ (237,586)
Noncontrolling Interest, Increase from Business Combination					$ 305,420															$ 305,420
Purchase of noncontrolling interest		$ 18,056			(315)				$ (175)								$ 18,056			(140)
Other	4,307					2,171							359			1,777
Ending Balance at Dec. 31, 2017	1,139,741					1,763,371				40,472		(303,873)	(859,009)		(10,486)	505,855					3,063		348
Consolidated net income (loss)	147,421												93,977			53,444
Other comprehensive gain, net of tax	(8,377)														(6,595)	(1,782)
Equity-based compensation	2,308					1,503										801					4
Dividends to common stockholders	(115,232)												(113,133)			(2,099)
Earnings distributions to limited partnership interests and other noncontrolling interests	(126,127)															(126,127)
Grant of exchangeability and redemption of limited partnership interests	130,997					84,730										46,194					73
Issuance of Class A common stock (net of costs)	312,259					244,872										67,127					260
Redemption of FPUs	(622)															(622)
Repurchase of Class A common stock	(9,711)											(7,612)				(2,099)
Forfeitures of restricted Class A common stock	(910)					711						(1,424)				(197)
Issuance of Class A common stock for acquisitions	5,481					8,413				(4,120)						1,178					10
Issuance of contingent shares and limited partnership interests in connection with acquisitions	1,653					1,296										357
Noncontrolling Interest Decrease From Business Combination					(4,897)															(4,897)
Purchase of noncontrolling interest					(100)															(100)
Cumulative effect of standard adoption | Revenue [Member]	18,690												16,387			2,303
Cumulative effect of standard adoption | Equity Investments [Member]													1,671		(2,293)	622
Issuance of exchangeable preferred partnership units					$ 174,779															$ 174,779
Other	1					2										(1)
Ending Balance at Jun. 30, 2018	$ 1,667,354					$ 2,104,898				$ 36,352		$ (312,909)	$ (860,107)		$ (19,374)	$ 714,736					$ 3,410		$ 348