Quarterly report pursuant to Section 13 or 15(d)

Financial Instruments - Amortized Cost and Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail)

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Financial Instruments - Amortized Cost and Fair Value of Available-for-Sale Securities by Contractual Maturity (Detail) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Amortized cost    
Maturing within one year, Amortized cost $ 339,803 $ 151,938
Maturing in one to five years, Amortized cost 73,015 12,855
Total available-for-sale securities, Amortized cost 412,818 164,793
Estimated Fair value    
Maturing within one year, Estimated fair value 339,500 151,852
Maturing in one to five years, Estimated fair value 72,813 12,790
Total available-for-sale securities, Estimated fair value $ 412,313 $ 164,642