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Fair Value of Financial Instruments - Summary of Composition of Short and Long Term Investments (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost $ 404,291 $ 448,492
Unrealized Gains   8
Unrealized Losses (812) (443)
Aggregate Fair Value 403,479 448,057
Commercial Paper [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 4,891 5,874
Unrealized Losses (6) (7)
Aggregate Fair Value 4,885 5,867
Corporate Bonds [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 83,648 81,947
Unrealized Losses (489) (279)
Aggregate Fair Value 83,159 81,668
US Government Agency Obligations [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 3,000 4,000
Unrealized Losses (9) (13)
Aggregate Fair Value 2,991 3,987
US Treasury Securities [Member]    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 312,752 356,671
Unrealized Gains   8
Unrealized Losses (308) (144)
Aggregate Fair Value $ 312,444 $ 356,535