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Fair Value of Financial Instruments - Summary of Contractual Maturities of Short and Long Term Investments (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair Value Disclosures [Abstract]    
Amortized Cost Basis, Due within one year $ 481,071 $ 416,932
Amortized Cost Basis, Due after 1 year and within 2 years 11,466 31,560
Amortized Cost 492,537 448,492
Aggregate Fair Value, Due within one year 480,761 416,663
Aggregate Fair Value, Due after 1 year and within 2 years 11,450 31,394
Aggregate Fair Value, Total $ 492,211 $ 448,057