Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Cantor [Member]	Smith Mack [Member]	Smith Mack [Member] Restricted Stock Units (RSUs) [Member]	Newmark [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Cantor [Member]	Additional Paid-in Capital [Member] Smith Mack [Member]	Additional Paid-in Capital [Member] Smith Mack [Member] Restricted Stock Units (RSUs) [Member]	Contingent Class A Common Stock [Member]	Contingent Class A Common Stock [Member] Smith Mack [Member] Restricted Stock Units (RSUs) [Member]	Treasury Stock [Member]	Retained Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Noncontrolling Interest in Subsidiaries [Member]	Noncontrolling Interest in Subsidiaries [Member] Cantor [Member]	Noncontrolling Interest in Subsidiaries [Member] Smith Mack [Member]	Noncontrolling Interest in Subsidiaries [Member] Smith Mack [Member] Restricted Stock Units (RSUs) [Member]	Noncontrolling Interest in Subsidiaries [Member] Newmark [Member]	Class A Common Stock [Member]	Class A Common Stock [Member] Smith Mack [Member]	Class A Common Stock [Member] Smith Mack [Member] Restricted Stock Units (RSUs) [Member]	Class B Common Stock [Member]
Beginning 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, 144,630 shares	(910)					711						(1,424)			(197)
Issuance of Class A common stock and RSUs 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
Purchase of noncontrolling interest					$ (100)														$ (100)
Noncontrolling Interest Decrease From Business Combination					(4,897)														(4,897)
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 in Newmark Partners, L.P.					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
Beginning Balance at Mar. 31, 2018	1,439,202					1,984,297				40,298		(303,873)	(837,753)	(8,754)	561,353					3,286			348
Consolidated net income (loss)	49,990												35,203		14,787
Other comprehensive gain, net of tax	(13,034)													(10,620)	(2,414)
Equity-based compensation	1,016					667									348					1
Dividends to common stockholders	(59,656)												(57,557)		(2,099)
Earnings distributions to limited partnership interests and other noncontrolling interests	(74,902)														(74,902)
Grant of exchangeability and redemption of limited partnership interests	77,064					49,255									27,764					45
Issuance of Class A common stock (net of costs)	81,209					63,600									17,538					71
Redemption of FPUs	(793)														(793)
Repurchase of Class A common stock	(9,711)											(7,612)			(2,099)
Forfeitures of restricted Class A common stock, 144,630 shares	(910)					711						(1,424)			(197)
Issuance of Class A common stock and RSUs for acquisitions	1,454					5,079				(3,946)					314					7
Issuance of contingent shares and limited partnership interests in connection with acquisitions	1,653					1,296									357
Issuance of exchangeable preferred partnership units in Newmark Partners, L.P.					$ 174,779														$ 174,779
Other	(7)					(7)
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
Beginning Balance at Dec. 31, 2018	863,174					2,208,221						(314,240)	(1,105,019)	(24,465)	94,801					3,417			459
Consolidated net income (loss)	108,939												75,479		33,460
Other comprehensive gain, net of tax	(2,235)													(1,925)	(310)
Equity-based compensation	4,643					3,119									1,521					3
Dividends to common stockholders	(95,320)												(95,320)
Earnings distributions to limited partnership interests and other noncontrolling interests	(52,841)														(52,841)
Grant of exchangeability and redemption of limited partnership interests	23,873					15,455									8,360					58
Issuance of Class A common stock (net of costs)	556					436									119					1
Redemption of FPUs	(21)														(21)
Repurchase of Class A common stock	(1,236)											(970)			(266)
Contributions of capital to and from Cantor for equity-based compensation		$ 42					$ (97)									$ 139
Issuance of Class A common stock and RSUs for acquisitions			$ 2,102					$ 1,669									$ 428				$ 5
Other	(230)					51									(281)
Ending Balance at Jun. 30, 2019	851,446					2,228,854						(315,210)	(1,124,860)	(26,390)	85,109					3,484			459
Beginning Balance at Mar. 31, 2019	878,646					2,210,033						(315,210)	(1,090,585)	(23,553)	94,063					3,439			459
Consolidated net income (loss)	21,765												13,611		8,154
Other comprehensive gain, net of tax	(3,222)													(2,837)	(385)
Equity-based compensation	2,625					1,759									865					1
Dividends to common stockholders	(47,886)												(47,886)
Earnings distributions to limited partnership interests and other noncontrolling interests	(25,653)														(25,653)
Grant of exchangeability and redemption of limited partnership interests	23,515					15,645									7,831					39
Issuance of Class A common stock (net of costs)	253					199									54
Redemption of FPUs	(11)														(11)
Contributions of capital to and from Cantor for equity-based compensation		$ 422					$ 174									$ 248
Issuance of Class A common stock and RSUs for acquisitions			$ 228					$ 198									$ 25				$ 5
Other	764					846									(82)
Ending Balance at Jun. 30, 2019	$ 851,446					$ 2,228,854						$ (315,210)	$ (1,124,860)	$ (26,390)	$ 85,109					$ 3,484			$ 459