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Benefit Plans - Status of Unexercised Stock Options (Detail) - $ / shares
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]      
Number of stock options, outstanding at beginning of year (in shares) 703,669 1,084,686 1,284,321
Number of stock options, granted (in shares) 42,857 76,327 65,972
Number of stock options, exercised (in shares) (238,370) (368,838) (201,320)
Number of stock options, forfeited (in shares) 0 (82,006) (62,287)
Number of stock options, expired (in shares) (500) (6,500) (2,000)
Number of stock options, outstanding at the end of year (in shares) 507,656 703,669 1,084,686
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]      
Weighted average exercise price, outstanding at beginning of year (in dollars per share) $ 14.70 $ 15.32 $ 15.32
Weighted average exercise price, granted (in dollars per share) 26.31 18.70 16.38
Weighted average exercise price, exercised (in dollars per share) 12.22 16.92 15.72
Weighted average exercise price, forfeited (in dollars per share) 0.00 16.42 14.93
Weighted average exercise price, expired (in dollars per share) 17.94 18.55 17.94
Weighted average exercise price, outstanding at the end of year (in dollars per share) $ 16.84 $ 14.70 $ 15.32