Equity - Schedule of Basic and Diluted Earnings Per Share, or EPS, Based on Weighted-Average of Both Restricted and Unrestricted Class A Common Stock Outstanding (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 57,891	$ 57,722	$ 50,613	$ 51,405	$ 53,375	$ 64,794	$ 63,081	$ 57,047	$ 217,631	$ 238,297	$ 196,829
Weighted-average shares outstanding, basic and diluted									95,963,616	94,165,351	81,740,227
Per share amount, basic and diluted	$ 0.59	$ 0.61	$ 0.53	$ 0.54	$ 0.57	$ 0.69	$ 0.67	$ 0.61	$ 2.27	$ 2.53	$ 2.41