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Stock-Based Compensation - Additional Information (Detail) - 2007 Plan [Member]
$ in Millions
3 Months Ended
Mar. 31, 2018
USD ($)
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Total unrecognized compensation cost for restricted stock granted | $ $ 5.4
Cost expected to be recognized over a weighted-average period 2 years 3 months 19 days
Minimum [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted stock units and restricted stock awards, vest, period 1 year
Maximum [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Restricted stock units and restricted stock awards, vest, period 5 years
Class A Common Stock [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Shares authorized | shares 7,500,000