Available-for-Sale Investments by Contractual Maturity (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 263,315
Amortized Cost, Due after one year through two years	148,275
Total	411,590
Estimated Fair Market Value, Due in one year or less	262,855
Estimated Fair Market Value, Due after one year through two years	147,589
Total	$ 410,444