Stock Transactions and Unit Redemptions - Schedule of Changes in Shares of Common Stock Outstanding (Detail) - shares	3 Months Ended				12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share issuances:
Other (in shares)					0
Common Class A
Increase (Decrease) in Stockholders' Equity
Shares outstanding at beginning of period (in shares)				156,265,461	156,265,461	156,916,336	138,593,787
Share issuances:
Issuance of Class A common stock in connection with The Spin-Off (in shares)					0	0	16,292,623
LPU redemption/exchange (in shares)					4,868,169	2,052,416	1,709,048
Other (in shares)						278,181	27,743
Treasury stock repurchases (in shares)	(930,226)	0	0	0	(930,226)	(4,518,002)	(50,000)
Shares outstanding at end of period (in shares)	161,175,894				161,175,894	156,265,461	156,916,336
Common Class A | Restricted Stock Units (RSUs)
Share issuances:
Issuance of Class A common stock for Newmark RSUs (in shares)					972,490	1,536,530	343,135