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Summary of Stock Option Activity (Detail) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Options outstanding, shares      
Beginning Balance 886 1,323 1,201
Granted 175 45 380
Exercised (12) (112) (91)
Forfeited/Expired (57) (370) (167)
Ending Balance 992 886 1,323
Exercisable as of December 31, 2017 536    
Expected to vest as of December 31, 2017 456    
Total 992    
Weighted Average Exercise Price Per Share      
Beginning Balance $ 143.89 $ 129.04 $ 111.77
Granted 169.04 189.62 178.93
Exercised 143.35 97.82 105.25
Forfeited/Expired 196.52 110.28 131.31
Ending Balance 145.28 $ 143.89 $ 129.04
Exercisable as of December 31, 2017 135.48    
Expected to vest as of December 31, 2017 156.79    
Total $ 145.28    
Aggregate Intrinsic Value      
Exercised $ 541 $ 8,102 $ 6,367
Exercisable as of December 31, 2017 3,941    
Expected to vest as of December 31, 2017 1,409    
Total $ 5,350    
Weighted Average Remaining Contractual Term (Years)      
Exercisable as of December 31, 2017 6 years 6 months    
Expected to vest as of December 31, 2017 7 years 10 months 24 days    
Total 7 years 2 months 12 days