Annual report pursuant to Section 13 and 15(d)

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

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Equity - Schedule of Issued Shares of Class A common stock Under 2009 Long-Term Incentive Plan (Detail) - 2009 Long-Term Incentive Plan [Member] - shares
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Schedule Of Employee Incentive Compensation Plans Disclosures [Line Items]    
Shares issued to employees upon exercise of stock options 45,800  
Class A Common Stock [Member] | Independent Directors Compensation Plan [Member]    
Schedule Of Employee Incentive Compensation Plans Disclosures [Line Items]    
Shares issued to independent directors for services rendered, in lieu of cash compensation 1,045 2,430
Class A Common Stock [Member] | Stock Option [Member]    
Schedule Of Employee Incentive Compensation Plans Disclosures [Line Items]    
Shares issued to employees upon exercise of stock options 45,800 15,000