Stock Transactions and Unit Redemptions - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015	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	$ 225,060,546
Redemptions, Number of Units	775,791
Reduction cost due to redemption and repurchases of shares	$ 65,900,000
Reduction cost due to redemption and repurchases of shares per share	$ 8.70
Class B Common Stock [Member]
Class of Stock [Line Items]
Common stock outstanding	34,848,000	34,848,107	34,848,000
Common stock, issued during the period	0	0
Common stock, shares issued	34,848,000		34,848,000
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock outstanding	236,750,000		219,063,000
Common stock, issued during the period	27,226		129,151
Maximum number of shares under the controlled equity offering	20,000,000
Common stock, shares issued	280,737,000		255,859,000
Company's repurchase and additional redemption authority amount				$ 300,000,000
Stock repurchase program and unit redemption remaining authorized amount	$ 225,100,000
Reduction in diluted shares due to redemption and repurchases of shares	7,600,000
Class A Common Stock [Member] | Controlled Equity Offerings [Member]
Class of Stock [Line Items]
Common stock, shares issued	6,831,023
Redemptions, Number of Units	400,000