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Summary of Stock Option Activity (Detail)
$ / shares in Units, shares in Thousands, $ in Thousands
 3 Months Ended
Mar. 31, 2018
USD ($)
$ / shares
shares
Options outstanding, shares  
Beginning Balance | shares 992
Granted | shares 475
Exercised | shares 0
Forfeited/Expired | shares (4)
Ending Balance | shares 1,463
Exercisable as of March 31, 2018 | shares 587
Expected to vest as of March 31, 2018 | shares 876
Total | shares 1,463
Weighted Average Exercise Price Per Share  
Beginning Balance | $ / shares $ 145.28
Granted | $ / shares 131.71
Exercised | $ / shares 0.00
Forfeited/Expired | $ / shares 165.01
Ending Balance | $ / shares 140.83
Exercisable as of March 31, 2018 | $ / shares 138.98
Expected to vest as of March 31, 2018 | $ / shares 142.06
Total | $ / shares $ 140.83
Aggregate Intrinsic Value  
Exercised | $ $ 0
Exercisable as of March 31, 2018 | $ 3,029
Expected to vest as of March 31, 2018 | $ 1,085
Total | $ $ 4,114
Weighted Average Remaining Contractual Term (Years)  
Exercisable as of March 31, 2018 6 years 6 months
Expected to vest as of March 31, 2018 8 years 9 months 18 days
Total 7 years 10 months 24 days