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Share-Based Compensation - Summary of Option Activity (Detail) - Option Activity Under the 2003 Plan [Member]
$ / shares in Units, $ in Thousands
 12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
shares
Schedule Of Stock Option Activity [Line Items]  
Option outstanding, Shares, Beginning balance | shares 36,635
Option granted, Shares | shares 0
Option Exercised, Shares | shares (9,185)
Option forfeited, Shares | shares 0
Option outstanding, Shares, Ending balance | shares 27,450
Option Exercisable, Shares, Ending balance | shares 27,450
Option outstanding, Weighted Average Exercise Price, Beginning balance | $ / shares $ 7.25
Option Granted, Weighted Average Exercise Price | $ / shares 0
Option Exercised, Weighted Average Exercise Price | $ / shares 6.86
Option Forfeited, Weighted Average Exercise Price | $ / shares 0
Option Outstanding, Weighted Average Exercise Price, Ending balance | $ / shares 7.37
Option Exercisable, Weighted Average Exercise Price, Ending balance | $ / shares $ 7.37
Option Outstanding, Weighted Average Remaining Contractual Term 2 years 8 months 23 days
Option Exercisable, Weighted Average Remaining Contractual Term 2 years 8 months 23 days
Option outstanding, Aggregate Intrinsic Value | $ $ 560,691
Option Exercisable, Aggregate Intrinsic Value | $ $ 560,691