Stock Transactions and Unit Redemptions - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jul. 30, 2014
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	$ 145,835,541
Class A Common Stock [Member]
Class of Stock [Line Items]
Common stock outstanding	185,108,000	181,583,000
Common stock, issued during the period	47,896	1,053,842	4,797,177
Maximum number of shares under the controlled equity offering	20,000,000
Common stock, shares issued	220,217,000	202,671,000
Company's repurchase and additional redemption authority amount				250,000,000
Stock repurchase program and unit redemption remaining authorized amount	$ 145,800,000
Class A Common Stock [Member] | Controlled Equity Offerings [Member]
Class of Stock [Line Items]
Common stock, shares issued	19,581,385
Class B Common Stock [Member]
Class of Stock [Line Items]
Common stock outstanding	34,848,000	34,848,000
Common stock, issued during the period	0	0
Common stock, shares issued	34,848,000	34,848,000