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Share-Based Compensation - Summary of Activity Related to Stock Options Granted under Company's Incentive Plan (Detail)
$ / shares in Units, $ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
USD ($)
$ / shares
shares
Jun. 30, 2018
USD ($)
$ / shares
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total Shares outstanding, Beginning balance   694,000
Total Shares, Granted   0
Total Shares, Exercised   (112,000)
Total Shares, Forfeited, canceled or expired   0
Total Shares outstanding, Ending balance 582,000 582,000
Total Shares, Exercisable 582,000 582,000
Employee Stock Option [Member]    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Total Shares, Granted 0  
Weighted Average Exercise Price per Share Outstanding, Beginning balance | $ / shares   $ 17.31
Weighted Average Exercise Price per Share, Granted | $ / shares   0
Weighted Average Exercise Price per Share, Exercised | $ / shares   15.72
Weighted Average Exercise Price per Share, Forfeited, canceled or expired | $ / shares   0
Weighted Average Exercise Price per Share Outstanding, Ending balance | $ / shares $ 17.62 17.62
Weighted Average Exercise Price per Share, Exercisable | $ / shares $ 17.62 $ 17.62
Weighted Average Remaining Contractual Term (Years), Outstanding   2 years 3 months 19 days
Weighted Average Remaining Contractual Term (Years), Exercisable   2 years 3 months 19 days
Aggregate Intrinsic Value, Outstanding | $ $ 54,657 $ 54,657
Aggregate Intrinsic Value, Exercisable | $ $ 54,657 $ 54,657