Annual report pursuant to Section 13 and 15(d)

Stock Based Compensation - Status of Unvested Shares of Stock Issued to Employees and Directors (Detail)

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Stock Based Compensation - Status of Unvested Shares of Stock Issued to Employees and Directors (Detail) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]      
Unvested at beginning of year, Non-vested Shares 258,163 305,520 310,463
Granted, Non-vested Shares 51,276 23,405 64,665
Vested, Non-vested Shares (96,615) (70,762) (69,187)
Forfeited, Non-vested Shares (42,015)   (421)
Unvested at end of year, Non-vested Shares 170,809 258,163 305,520
Unvested at beginning of year, Weighted average grant date fair value $ 58.89 $ 59.09 $ 51.93
Granted, Weighted average grant date fair value 85.17 89.30 94.74
Vested, Weighted average grant date fair value 58.95 69.82 60.28
Forfeited, Weighted average grant date fair value 38.53   76.07
Unvested at end of year, Weighted average grant date fair value $ 71.75 $ 58.89 $ 59.09