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Fair Value of Financial Instruments - Summary of Contractual Maturities of Short and Long Term Investments (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Fair Value Disclosures [Abstract]    
Amortized Cost Basis, Due within one year $ 315,178 $ 54,694
Amortized Cost Basis, Due after 1 year through 2 years 35,714 35,876
Amortized Cost 350,892 90,570
Aggregate Fair Value, Due within one year 315,130 54,648
Aggregate Fair Value, Due after 1 year through 2 years 35,669 35,718
Aggregate Fair Value, Total $ 350,799 $ 90,366