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
Sep. 30, 2018
Dec. 31, 2017
Amortized cost    
Maturing within one year, Amortized cost $ 324,519 $ 151,938
Maturing in one to five years, Amortized cost 39,202 12,855
Total available-for-sale securities, Amortized cost 363,721 164,793
Estimated Fair value    
Maturing within one year, Estimated fair value 324,204 151,852
Maturing in one to five years, Estimated fair value 39,067 12,790
Total available-for-sale securities, Estimated fair value $ 363,271 $ 164,642