Stock Transactions and Unit Redemptions - Additional Information (Detail) - USD ($)			3 Months Ended			6 Months Ended		12 Months Ended
	Mar. 09, 2018	Apr. 12, 2017	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Feb. 07, 2017
Class of Stock [Line Items]
Payment percentage of the gross proceeds from the sale of shares	2.00%	2.00%
Stock repurchase program and unit redemption remaining authorized amount			$ 106,137,021			$ 106,137,021
Redemptions, Number of Units			4,299,936	1,723,654		6,023,590
Class B Common Stock [Member]
Class of Stock [Line Items]
Common stock, issued during the period			0		0	0	0
Common stock outstanding			34,848,107		34,848,107	34,848,107	34,848,107	34,848,000
Common stock, shares issued			34,848,000			34,848,000		34,848,000
Common stock value issued			$ 348,000			$ 348,000		$ 348,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, issued during the period						25,951,473		1,994,670
Common stock outstanding			290,910,000			290,910,000		256,968,000
Common stock, shares issued			341,030,000			341,030,000		306,218,000
Common stock value issued			$ 3,410,000			$ 3,410,000		$ 3,063,000
Company's repurchase and additional redemption authority amount									$ 300,000,000
Stock repurchase program and unit redemption remaining authorized amount			$ 106,100,000			$ 106,100,000
Class A Common Stock [Member] | April 2017 Sales Agreement
Class of Stock [Line Items]
Maximum number of shares under the controlled equity offering		20,000,000
Class A Common Stock [Member] | Controlled Equity Offerings [Member]
Class of Stock [Line Items]
Common stock, shares issued			20,000,000			20,000,000
Redemptions, Number of Units						6,000,000
Class A Common Stock [Member] | March 2019 Sales Agreement
Class of Stock [Line Items]
Maximum number of shares under the controlled equity offering	300,000,000
Class A Common Stock [Member] | New Controlled Equity Offerings [Member]
Class of Stock [Line Items]
Common stock, shares issued			13,352,311			13,352,311
Common stock value issued			$ 168,400,000			$ 168,400,000