Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Additional Paid-in Capital [Member]	Retained Earnings [Member]	BGC’s Net Investment in Newmark [Member]	Noncontrolling Interests in Subsidiaries [Member]	Class A Common Stock [Member]	Class B Common Stock [Member]
Beginning Balance at Dec. 31, 2016	$ 983,783		$ 245,877	$ 735,899	$ 2,007
Consolidated net income (loss)	145,096		144,492		604
Distributions	(190,948)		(190,877)		(71)
Purchase of noncontrolling interests				1,092	(1,092)
Noncontrolling interests in an entity acquired	19,146				19,146
Debt assumed from BGC	(1,387,500)			(1,387,500)
Contributions	368,418			368,418
Transfer of pre initial public offering (“IPO”) capital to redeemable partnership interests	(21,096)			(21,096)
Issuance of shares in the Separation (Class A common stock, 115,593,787 shares); (Class B common stock, 15,840,049 shares)		$ (245,815)		$ 303,187	(58,686)	$ 1,156	$ 158
Proceeds from IPO, net of underwriting discounts and other expenses (Class A common stock, 23,000,000 shares)	295,419	295,189				230
Equity-based compensation and related issuance	10,000	10,000
Ending Balance at Dec. 31, 2017	222,318	59,374	199,492		(38,092)	1,386	158
Cumulative effect of revenue standard adoption	18,805		16,463		2,342
Consolidated net income (loss)	32,487		19,997		12,490
Reduction of earnings distributions	2,144		2,144
Distributions	(100)				(100)
BGC's purchase of 16,600,000 exchangeable limited partnership unitsin Newmark Holdings	241,960				241,960
Grant of exchangeability and redemption of limitedpartnership interests	19,869				19,869
Allocation of net income to redeemable partnership interests	(1,009)				(1,009)
Equity-based compensation and related issuance	(4,897)	(4,900)				3
Ending Balance at Mar. 31, 2018	$ 531,577	$ 54,474	$ 238,096		$ 237,460	$ 1,389	$ 158