Stock Transactions and Unit Redemptions - Additional Information (Detail) - USD ($)	3 Months Ended			6 Months Ended		12 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	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	$ 201,785,151			$ 201,785,151
Redemptions, Number of Units		775,791		1,804,365
Reduction cost due to redemption and repurchases of shares				$ 74,500,000
Reduction cost due to redemption and repurchases of shares per share				$ 8.73
Class B Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares authorized	150,000,000			150,000,000		100,000,000	150,000,000	100,000,000
Common stock outstanding	34,848,000		34,848,107	34,848,000	34,848,107	34,848,000
Common stock, issued during the period	0		0	0	0
Common stock, shares issued	34,848,000			34,848,000		34,848,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock, shares authorized	750,000,000			750,000,000		500,000,000	750,000,000	500,000,000
Common stock outstanding	241,292,000			241,292,000		219,063,000
Common stock, issued during the period				1,690,305		129,151
Maximum number of shares under the controlled equity offering				20,000,000
Common stock, shares issued	286,088,000			286,088,000		255,859,000
Company's repurchase and additional redemption authority amount									$ 300,000,000
Stock repurchase program and unit redemption remaining authorized amount	$ 201,800,000			$ 201,800,000
Reduction in diluted shares due to redemption and repurchases of shares				8,500,000
Class A Common Stock [Member] | Controlled Equity Offerings [Member]
Class of Stock [Line Items]
Common stock, shares issued	10,128,575			10,128,575
Redemptions, Number of Units				2,000,000