Stock Transactions and Unit Redemptions - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended
	Apr. 12, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Aug. 01, 2018	Mar. 31, 2018
Class of Stock [Line Items]
Payment percentage of the gross proceeds from the sale of shares	2.00%
Stock repurchase program and unit redemption remaining authorized amount			$ 256,882,218		$ 256,882,218
Class B Common Stock [Member]
Class of Stock [Line Items]
Common stock, issued during the period		0	0	0	0
Common stock outstanding		45,884,380		45,884,380		45,884,380
Common stock, shares issued		45,884,000		45,884,000		45,884,000
Common stock value issued		$ 459,000		$ 459,000		$ 459,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, issued during the period		53,810	23,767	162,237	25,975,240
Common stock outstanding		302,315,000		302,315,000		291,475,000
Common stock, shares issued		352,840,000		352,840,000		341,745,000
Common stock value issued		$ 3,528,000		$ 3,528,000		$ 3,417,000
Company's repurchase and additional redemption authority amount							$ 300,000,000.0
Stock repurchase program and unit redemption remaining authorized amount		$ 256,900,000		$ 256,900,000
Class A Common Stock [Member] | Controlled Equity Offerings [Member]
Class of Stock [Line Items]
Common stock, shares issued								20,000,000
Class A Common Stock [Member] | New Controlled Equity Offerings [Member]
Class of Stock [Line Items]
Common stock, shares issued		17,401,431		17,401,431
Common stock value issued		$ 209,800,000		$ 209,800,000