Stock Transactions and Unit Redemptions - Schedule of Changes in Shares of Common Stock Outstanding (Detail) - shares	3 Months Ended				9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Share issuances:
Other (in shares)	0				0
Class A Common Stock
Increase (Decrease) in Stockholders' Equity
Shares outstanding at beginning of period (in shares)	157,811,436		156,265,461	156,679,527	156,265,461	156,916,336
Share issuances:
LPU redemption/exchange (in shares)	607,971			543,990	1,617,690	1,666,766
Other (in shares)				2,364		81,206
Treasury stock repurchases (in shares)	0	0	0	(2,279,373)	0	(3,892,405)
Shares outstanding at end of period (in shares)	158,630,125	157,811,436		154,989,126	158,630,125	154,989,126
Class A Common Stock | Restricted Stock Units (RSUs)
Share issuances:
Issuance of Class A common stock for Newmark RSUs (in shares)	210,718			42,618	746,974	217,223