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Fair Value of Financial Instruments - Summary of Contractual Maturities of Short and Long Term Investments (Detail) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Fair Value Disclosures [Abstract]    
Amortized Cost Basis, Due within one year $ 375,946 $ 416,932
Amortized Cost Basis, Due after 1 year through 2 years 28,345 31,560
Amortized Cost 404,291 448,492
Aggregate Fair Value, Due within one year 375,379 416,663
Aggregate Fair Value, Due after 1 year through 2 years 28,100 31,394
Aggregate Fair Value, Total $ 403,479 $ 448,057