Equity - Schedule of Movement in Outstanding Shares of Class A Common Stock, Restricted Class A Common Stock and Deferred Stock Units (Detail) - shares	1 Months Ended	9 Months Ended
	May 31, 2013	Sep. 30, 2018	Sep. 30, 2017
Equity [Abstract]
Beginning balance		108,081,077	94,709,290
Issuance of class A common stock	25,875,000	11,484,414	971
Issuance of restricted class A common stock, net		300,921	286,773
Issuance of deferred stock units		23,730	20,560
Ending balance		119,890,142	95,017,594