Equity - Summary of Class A Common Stock Issuances (Detail) - USD ($)	1 Months Ended	9 Months Ended
	May 31, 2013	Sep. 30, 2019	Sep. 30, 2018
Class of Stock [Line Items]
Shares issued	25,875,000	10,535,507	11,484,414
Net proceeds		$ 372,329,000	$ 369,787,000
Shares issued, Wtd. Avg.		10,534,628
Gross share issue price, Wtd. Avg.		$ 35.75
Net share issue price, Wtd. Avg.		$ 35.38
Net proceeds,Wtd. Avg.		$ 372,341
Class A Common Stock [Member]
Class of Stock [Line Items]
Shares issued		8,625,000
Gross share issue price		$ 36.00
Net share issue price		$ 35.62
Net proceeds		$ 306,952
Class A Common Stock [Member] | At The Market [Member]
Class of Stock [Line Items]
Shares issued		1,909,628
Gross share issue price		$ 34.63
Net share issue price		$ 34.28
Net proceeds		$ 65,389