Quarterly report pursuant to Section 13 or 15(d)

Equity and Share-Based Compensation - Additional Information (Detail)

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Equity and Share-Based Compensation - Additional Information (Detail) - USD ($)
6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Stock compensation expense $ 714,000 $ 848,000
Options exercised during period 45,800  
Proceeds from stock options exercised $ 258,000  
2009 Long-Term Incentive Plan [Member] | Class A Common Stock [Member] | Independent Directors Compensation Plan [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Issuance of shares for compensation 1,045