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Stock Plans - Summary of Performance Stock Unit Activity (Detail) - Performance Stock Unit Activity - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Unvested Shares, Beginning Balance 171,644 186,621 171,644 143,961
Granted, Shares   98,886 65,355 87,256
Vested, Shares   (72,504) (31,093) (52,839)
Performance adjustments, Shares   29,183 (15,682) 3,086
Forfeited, Shares (3,603)     (9,820)
Unvested Shares, Ending Balance   242,186 186,621 171,644
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]        
Unvested Weighted Average Grant Date Fair Value, Beginning Balance (dollars per share) $ 50.24 $ 46.18 $ 50.24 $ 52.42
Granted, Weighted Average Grant Date Fair Value (dollars per share)   72.73 44.28 41.99
Vested, Weighted Average Grant Date Fair Value (dollars per share)   57.19 58.54 41.75
Performance adjustments, Weighted Average Grant Date Fair value (dollars per share)   57.27 58.54 41.45
Forfeited, Weighted Average Grant Date Fair Value (dollars per share) $ 44.47     51.51
Unvested Weighted Average Grant Date Fair Value, Ending Balance (dollars per share)   $ 55.06 $ 46.18 $ 50.24