Stock Transactions and Unit Redemptions - Schedule of Changes in Shares of Common Stock Outstanding (Detail) - shares	3 Months Ended			6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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)	156,701,746	156,265,461	157,422,916	156,265,461	156,916,336
Share issuances:
LPU redemption/exchange (in shares)	827,847		747,846	1,009,719	1,122,776
Other (in shares)			70,391		78,842
Treasury stock repurchases (in shares)	0	0	(1,613,032)	0	(1,613,032)
Shares outstanding at end of period (in shares)	157,811,436	156,701,746	156,679,527	157,811,436	156,679,527
Class A Common Stock | Restricted Stock Units (RSUs)
Share issuances:
Issuance of Class A common stock for Newmark RSUs (in shares)	281,843		51,406	536,256	174,605