Stock Transactions and Unit Redemptions - Additional Information (Detail) - USD ($)		3 Months Ended			6 Months Ended		12 Months Ended
	Apr. 12, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016	Jun. 22, 2016	Jun. 21, 2016	Oct. 27, 2015
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		$ 253,618,546			$ 253,618,546
Redemptions, Number of Units		1,908,867	2,927,758		4,800,000
Reduction cost due to redemption and repurchases of shares					$ 16,800,000
Reduction cost due to redemption and repurchases of shares per share					$ 11.08
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares authorized		750,000,000			750,000,000		750,000,000	750,000,000	500,000,000
Common stock, issued during the period					33,616		2,004,533
Common stock outstanding		251,057,000			251,057,000		244,870,000
Maximum number of shares under the controlled equity offering					20,000,000
Common stock, shares issued		299,722,000			299,722,000		292,549,000
Company's repurchase and additional redemption authority amount										$ 300,000,000
Stock repurchase program and unit redemption remaining authorized amount		$ 253,600,000			$ 253,600,000
Reduction in diluted shares due to redemption and repurchases of shares					1,500,000
Class A Common Stock [Member] | Controlled Equity Offerings [Member]
Class of Stock [Line Items]
Common stock, shares issued		18,314,294			18,314,294
Redemptions, Number of Units					4,300,000
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
Common stock, shares issued	0	0			0
Class B Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares authorized		150,000,000			150,000,000		150,000,000	150,000,000	100,000,000
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