Annual report pursuant to Section 13 and 15(d)

Schedule of Issued Shares of Class A common stock Under 2009 Long-Term Incentive Plan (Detail)

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Schedule of Issued Shares of Class A common stock Under 2009 Long-Term Incentive Plan (Detail) - 2009 Long-Term Incentive Plan [Member] - Class A Common Stock - shares
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Independent Directors Compensation Plan [Member]      
Schedule Of Employee Incentive Compensation Plans Disclosures [Line Items]      
Restricted shares issued to independent directors for services rendered, in lieu of cash compensation 16,887 19,542 49,187
Stock Option      
Schedule Of Employee Incentive Compensation Plans Disclosures [Line Items]      
Shares issued to employees upon exercise of stock options 33,825 354,926 160,470