Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Class A Common Stock [Member]	Class B Common Stock [Member]	Contingent Class A Common Stock	Additional Paid-in Capital [Member]	Treasury Stock [Member]	Retained Earnings [Member]	Noncontrolling Interests in Subsidiaries [Member]	Common Stock
Beginning Balance at Dec. 31, 2017	$ 222,318	$ 1,386	$ 158		$ 59,374		$ 199,492	$ (38,092)
Consolidated net income	152,339						88,973	63,366
Cumulative effect of revenue standard adoption	18,805						16,463	2,342
Reduction of earning distributions	2,144						2,144
Dividends to common stockholders	(27,857)						(27,857)
Preferred dividend on exchangeable preferred partnership units	(1,866)						(1,866)
Earning distributions to limited partnership interests, redeemable partnership interests, and other noncontrolling interests	(42,962)							(42,962)
Equity-based compensation and related issuance of Class A common stock	(4,897)	3			(4,900)
Contingent Class A common stock to be issued to newly acquired entities	3,136				3,136
BGC's purchase of exchangeable limited partnership units in Newmark Holdings	241,960							241,960
Grant of exchangeability, redemption and issuance of limited partnership interests and issuance of common stock	93,943							93,943
Issuance of exchangeable preferred partnership units in Newmark OpCo	322,653							322,653
Issuance and redemption of limited partnership units including contingent units	11,115							11,115
Other	(2,531)				(1,882)			(649)
Ending Balance at Sep. 30, 2018	988,300	1,389	158		55,728		277,349	653,676
Dividends declared per share of common stock									$ 0.27
Dividends paid per share of common stock									0.18
Beginning Balance at Jun. 30, 2018	757,395	1,389	158		60,294		224,906	470,648
Consolidated net income	115,558						68,237	47,321
Dividends to common stockholders	(13,928)						(13,928)
Preferred dividend on exchangeable preferred partnership units	(1,866)						(1,866)
Earning distributions to limited partnership interests, redeemable partnership interests, and other noncontrolling interests	(29,249)							(29,249)
Contingent Class A common stock to be issued to newly acquired entities	3,136				3,136
Grant of exchangeability, redemption and issuance of limited partnership interests and issuance of common stock	9,666							9,666
Issuance of exchangeable preferred partnership units in Newmark OpCo	147,874							147,874
Issuance and redemption of limited partnership units including contingent units	9,462							9,462
Other	(9,748)				(7,702)			(2,046)
Ending Balance at Sep. 30, 2018	988,300	1,389	158		55,728		277,349	653,676
Dividends declared per share of common stock									0.09
Dividends paid per share of common stock									0.09
Beginning Balance at Dec. 31, 2018	1,056,799	1,570	212	$ 3,250	285,071	$ (486)	277,952	489,230
Consolidated net income	178,039						128,270	49,769
Dividends to common stockholders	(51,659)						(51,659)
Preferred dividend on exchangeable preferred partnership units							(9,672)	9,672
Earning distributions to limited partnership interests, redeemable partnership interests, and other noncontrolling interests	(67,622)							(67,622)
Grant of exchangeability, redemption and issuance of limited partnership interests and issuance of common stock	(13,180)	20			7,632			(20,832)
Issuance and redemption of limited partnership units including contingent units	2,715				914			1,801
Repurchase of common stock	(33,983)					(33,983)
Redemption of EPUs	(93,480)							(93,480)
Other	1,284				1,284
Ending Balance at Sep. 30, 2019	978,913	1,590	212	3,250	294,901	(34,469)	344,891	368,538
Dividends declared per share of common stock									0.30
Dividends paid per share of common stock									0.29
Beginning Balance at Jun. 30, 2019	1,016,003	1,583	212	2,956	292,441	(14,382)	277,115	456,078
Consolidated net income	122,569						88,698	33,871
Dividends to common stockholders	(17,698)						(17,698)
Preferred dividend on exchangeable preferred partnership units							(3,224)	3,224
Earning distributions to limited partnership interests, redeemable partnership interests, and other noncontrolling interests	(26,101)							(26,101)
Grant of exchangeability, redemption and issuance of limited partnership interests and issuance of common stock	682	7			2,872			(2,197)
Issuance and redemption of limited partnership units including contingent units	(4,306)				(1,449)			(2,857)
Repurchase of common stock	(20,087)					(20,087)
Redemption of EPUs	(93,480)							(93,480)
Other	1,331			294	1,037
Ending Balance at Sep. 30, 2019	$ 978,913	$ 1,590	$ 212	$ 3,250	$ 294,901	$ (34,469)	$ 344,891	$ 368,538
Dividends declared per share of common stock									0.10
Dividends paid per share of common stock									$ 0.10