Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Cantor [Member]	Cumulative Effect Period of Adoption Adjustment [Member]	Smith Mack [Member]	Newmark [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Cantor [Member]	Additional Paid-in Capital [Member] Smith Mack [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] Cumulative Effect Period of Adoption Adjustment [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Other Comprehensive Income (Loss) [Member] Cumulative Effect Period of Adoption Adjustment [Member]	Noncontrolling Interest in Subsidiaries [Member]	Noncontrolling Interest in Subsidiaries [Member] Cantor [Member]	Noncontrolling Interest in Subsidiaries [Member] Cumulative Effect Period of Adoption Adjustment [Member]	Noncontrolling Interest in Subsidiaries [Member] Smith Mack [Member]	Noncontrolling Interest in Subsidiaries [Member] Newmark [Member]	Class A Common Stock [Member] Common Stock [Member]	Class A Common Stock [Member] Common Stock [Member] Smith Mack [Member]	Class B Common Stock [Member] Common Stock [Member]
Beginning Balance at Dec. 31, 2017	$ 1,127,870					$ 1,764,013			$ 40,472		$ (303,873)	$ (866,875)		$ (10,486)		$ 501,208					$ 3,063		$ 348
Consolidated net income (loss)	286,610											202,196				84,414
Other comprehensive income (loss), net of tax	(13,543)													(11,686)		(1,857)
Equity-based compensation	7,466					4,242										3,219					5
Dividends to common stockholders	(237,746)											(231,446)				(6,300)
Earnings distributions to limited partnership interests and other noncontrolling interests	(201,484)															(201,484)
Grant of exchangeability and redemption of limited partnership interests	238,652					158,913										79,556					183
Issuance of Class A common stock (net of costs)	312,842					245,338										67,244					260
Exchange Class A to Class B common stock																					(111)		111
Redemption of FPUs	(925)															(925)
Repurchase of Class A common stock	(10,439)										(8,185)					(2,254)
Forfeitures of restricted Class A common stock	(1,457)					1,031					(2,182)					(306)
Contributions of capital to and from Cantor for equity-based compensation	(30,174)					(15,921)						(3,932)				(10,321)
Issuance of Class A common stock and RSUs for acquisitions				$ 7,648				$ 46,454		$ (40,472)									$ 1,649			$ 17
Issuance of contingent shares and limited partnership interests in connection with acquisitions	14,251															14,251
Purchase of noncontrolling interest					$ (54)															$ (54)
Noncontrolling Interest, Increase from Business Combination					$ 2,053															$ 2,053
Issuance of EPUs	320,786															320,786
Spin-Off of Newmark	(990,313)											(226,210)				(764,103)
Other	(952)					(617)										(335)
Ending Balance at Dec. 31, 2018	849,781					2,203,453					(314,240)	(1,108,209)		(24,465)		89,366					3,417		459
Ending Balance (Revenue [Member]) at Dec. 31, 2018			$ 18,690										$ 16,387					$ 2,303
Ending Balance (Equity Investments [Member]) at Dec. 31, 2018													$ 1,671		$ (2,293)			$ 622
Consolidated net income (loss)	72,253											47,562				24,691
Other comprehensive income (loss), net of tax	(13,014)													(8,637)		(4,377)
Equity-based compensation	10,489					7,066										3,419					4
Dividends to common stockholders	(192,442)											(192,442)
Earnings distributions to limited partnership interests and other noncontrolling interests	(89,616)															(89,616)
Grant of exchangeability and redemption of limited partnership interests	86,458					56,455										29,853					150
Issuance of Class A common stock (net of costs)	1,116					879										235					2
Redemption of FPUs	(256)															(256)
Repurchase of Class A common stock	(1,236)										(970)					(266)
Forfeiture of Class A common stock	(139)					(12)					(98)					(29)
Contributions of capital to and from Cantor for equity-based compensation		$ (136)					$ (382)										$ 246
Issuance of Class A common stock and RSUs for acquisitions	3,040					4,471										(1,442)					11
Other	(1,330)					173										(1,503)
Ending Balance at Dec. 31, 2019	724,968					2,272,103					(315,308)	(1,253,089)		(33,102)		50,321					3,584		459
Consolidated net income (loss)	20,169											13,674				6,495
Ending Balance at Mar. 31, 2020	696,324					2,293,221						(1,289,931)				48,349
Beginning Balance at Dec. 31, 2019	724,968					2,272,103					(315,308)	(1,253,089)		(33,102)		50,321					3,584		459
Consolidated net income (loss)	59,442											41,593				17,849
Ending Balance at Jun. 30, 2020	742,760					2,309,129						(1,265,589)				57,001
Beginning Balance at Dec. 31, 2019	724,968					2,272,103					(315,308)	(1,253,089)		(33,102)		50,321					3,584		459
Consolidated net income (loss)	69,346											51,246				18,100
Ending Balance at Sep. 30, 2020	767,681					2,317,862						(1,259,540)				59,960
Beginning Balance at Dec. 31, 2019	724,968					2,272,103					(315,308)	(1,253,089)		(33,102)		50,321					3,584		459
Consolidated net income (loss)	56,602											48,908				7,694
Other comprehensive income (loss), net of tax	4,617													4,172		445
Equity-based compensation	12,672					8,565										4,096					11
Dividends to common stockholders	(60,440)											(60,440)
Earnings distributions to limited partnership interests and other noncontrolling interests	(36,569)															(36,569)
Grant of exchangeability and redemption of limited partnership interests	93,793					61,766										31,895					132
Issuance of Class A common stock (net of costs)	5,505					5,381										120					4
Redemption of FPUs	(102)															(102)
Repurchase of Class A common stock	(6)										(5)					(1)
Contributions of capital to and from Cantor for equity-based compensation		$ 5,519					$ 3,613										$ 1,906
Issuance of Class A common stock and RSUs for acquisitions				$ 1,578				$ 1,664											$ (90)			$ 4
Other	1,392					1,400										(8)
Cumulative effect of CECL standard adoption | Accounting Standards Update 2016-13 [Member]	(1,300)											(883)				(417)
Ending Balance at Dec. 31, 2020	808,229					2,354,492					(315,313)	(1,265,504)		(28,930)		59,290					3,735		459
Beginning Balance at Mar. 31, 2020	696,324					2,293,221						(1,289,931)				48,349
Consolidated net income (loss)	39,273											27,919				11,354
Ending Balance at Jun. 30, 2020	742,760					2,309,129						(1,265,589)				57,001
Consolidated net income (loss)	9,904											9,653				251
Ending Balance at Sep. 30, 2020	767,681					2,317,862						(1,259,540)				59,960
Consolidated net income (loss)	(12,744)
Ending Balance at Dec. 31, 2020	$ 808,229					$ 2,354,492					$ (315,313)	$ (1,265,504)		$ (28,930)		$ 59,290					$ 3,735		$ 459