Condensed Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Total	Cantor [Member]	Smith Mack [Member]	Additional Paid-in Capital [Member]	Additional Paid-in Capital [Member] Cantor [Member]	Additional Paid-in Capital [Member] Smith Mack [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]	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, 2018	$ 863,174			$ 2,208,221			$ (314,240)	$ (1,105,019)	$ (24,465)	$ 94,801			$ 3,417		$ 459
Consolidated net income (loss)	111,499							71,950		39,549
Other comprehensive income (loss), net of tax	(10,476)								(9,187)	(1,289)
Equity-based compensation	5,153			3,462						1,687			4
Dividends to common stockholders	(143,673)							(143,673)
Earnings distributions to limited partnership interests and other noncontrolling interests	(71,310)									(71,310)
Grant of exchangeability and redemption of limited partnership interests	42,045			27,388						14,560			97
Issuance of Class A common stock (net of costs)	825			648						175			2
Redemption of FPUs	(190)									(190)
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		$ 961			$ 464						$ 497
Issuance (reversal) of Class A common stock and RSUs for acquisitions			$ 3,038			$ 3,573						$ (543)		$ 8
Other	(132)			145				1		(278)
Ending Balance at Sep. 30, 2019	799,539			2,243,889			(315,308)	(1,176,741)	(33,652)	77,364			3,528		459
Beginning Balance at Jun. 30, 2019	851,446			2,228,854			(315,210)	(1,124,860)	(26,390)	85,109			3,484		459
Consolidated net income (loss)	2,560							(3,529)		6,089
Other comprehensive income (loss), net of tax	(8,241)								(7,262)	(979)
Equity-based compensation	510			343						166			1
Dividends to common stockholders	(48,353)							(48,353)
Earnings distributions to limited partnership interests and other noncontrolling interests	(18,469)									(18,469)
Grant of exchangeability and redemption of limited partnership interests	18,172			11,933						6,200			39
Issuance of Class A common stock (net of costs)	269			212						56			1
Redemption of FPUs	(169)									(169)
Forfeiture of Class A common stock	(139)			(12)			(98)			(29)
Contributions of capital to and from Cantor for equity-based compensation		919			561						358
Issuance (reversal) of Class A common stock and RSUs for acquisitions			936			1,904						(971)		3
Other	98			94				1		3
Ending Balance at Sep. 30, 2019	799,539			2,243,889			(315,308)	(1,176,741)	(33,652)	77,364			3,528		459
Beginning Balance at Dec. 31, 2019	743,737			2,271,947			(315,308)	(1,241,754)	(33,102)	57,911			3,584		459
Consolidated net income (loss)	85,521							61,794		23,727
Other comprehensive income (loss), net of tax	(6,139)								(6,303)	164
Equity-based compensation	9,816			6,631						3,176			9
Dividends to common stockholders	(56,814)							(56,814)
Earnings distributions to limited partnership interests and other noncontrolling interests	(28,285)									(28,285)
Grant of exchangeability and redemption of limited partnership interests	48,286			31,785						16,443			58
Issuance of Class A common stock (net of costs)	5,236			5,155						77			4
Redemption of FPUs	(34)									(34)
Repurchase of Class A common stock	(6)						(5)			(1)
Contributions of capital to and from Cantor for equity-based compensation		1,442			849						593
Issuance (reversal) of Class A common stock and RSUs for acquisitions			1,377			1,305						69		3
Cumulative effect of CECL standard adoption | Accounting Standards Update 2016-13 [Member]	(1,300)							(883)		(417)
Other	(212)			34						(246)
Ending Balance at Sep. 30, 2020	802,625			2,317,706			(315,313)	(1,237,657)	(39,405)	73,177			3,658		459
Beginning Balance at Jun. 30, 2020	762,678			2,308,973			(315,308)	(1,253,434)	(46,570)	64,920			3,638		459
Consolidated net income (loss)	24,930							19,381		5,549
Other comprehensive income (loss), net of tax	8,190								7,165	1,025
Equity-based compensation	3,999			2,701						1,297			1
Dividends to common stockholders	(3,604)							(3,604)
Earnings distributions to limited partnership interests and other noncontrolling interests	(3,142)									(3,142)
Grant of exchangeability and redemption of limited partnership interests	9,531			5,660						3,854			17
Issuance of Class A common stock (net of costs)	240			233						6			1
Redemption of FPUs	(32)									(32)
Repurchase of Class A common stock	(6)						(5)			(1)
Contributions of capital to and from Cantor for equity-based compensation		$ 137			$ 85						$ 52
Issuance (reversal) of Class A common stock and RSUs for acquisitions			$ (21)			$ 84						$ (106)		$ 1
Other	(275)			(30)						(245)
Ending Balance at Sep. 30, 2020	$ 802,625			$ 2,317,706			$ (315,313)	$ (1,237,657)	$ (39,405)	$ 73,177			$ 3,658		$ 459