Stock Transactions and Unit Redemptions - Schedule of Changes in Shares of Common Stock Outstanding (Detail) - Class A Common Stock - shares	3 Months Ended				12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shares outstanding at beginning of period (in shares)				156,916,336	156,916,336	138,593,787
Share issuances:
Issuance of Class A common stock in connection with The Spin-Off (in shares)					0	16,292,623
LPU redemption/exchange (in shares)					2,052,416	1,709,048
Other issuances of Class A common stock (in shares)					1,536,530	343,135
Issuance of Class A common stock for Newmark RSUs (in shares)							23,000,000
Treasury stock repurchases (in shares)	(625,597)	(2,279,373)	(1,613,032)	0	(4,518,002)	(50,000)
Shares outstanding at end of period (in shares)	156,265,461				156,265,461	156,916,336	138,593,787
Restricted Stock Units (RSUs)
Share issuances:
Issuance of Class A common stock for Newmark RSUs (in shares)					278,181	27,743