Equity - Summary of Class A Common Stock Issuances (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		9 Months Ended
	Aug. 31, 2018	May 31, 2013	Sep. 30, 2018	Sep. 30, 2017
Class of Stock [Line Items]
Shares issued		25,875,000	11,484,414	971
Share issue price			$ 32.73
Net proceeds			$ 369,787	$ 31
Class A Common Stock [Member]
Class of Stock [Line Items]
Shares issued	6,900,000		11,483,135
Share issue price	$ 32.60		$ 32.65
Net proceeds	$ 222,275		$ 369,787
Class A Common Stock [Member] | ATM Agreements [Member]
Class of Stock [Line Items]
Shares issued			4,583,135	0
Share issue price			$ 32.73
Net proceeds			$ 147,512