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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, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]        
Unvested Shares, Beginning Balance 186,621 242,186 186,621 171,644
Granted, Shares   62,775 98,886 65,355
Vested, Shares   (41,660) (72,504) (31,093)
Performance adjustments, Shares   (35,756) 29,183 (15,682)
Forfeited, Shares 0 (8,659)   (3,603)
Unvested Shares, Ending Balance   218,886 242,186 186,621
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) $ 46.18 $ 55.06 $ 46.18 $ 50.24
Granted, Weighted Average Grant Date Fair Value (dollars per share)   84.79 72.73 44.28
Vested, Weighted Average Grant Date Fair Value (dollars per share)   40.23 57.19 58.54
Performance adjustments, Weighted Average Grant Date Fair value (dollars per share)   45.21 57.27 58.54
Forfeited, Weighted Average Grant Date Fair Value (dollars per share) $ 0.00 44.53   44.47
Unvested Weighted Average Grant Date Fair Value, Ending Balance (dollars per share)   $ 68.43 $ 55.06 $ 46.18