Investments - Cost and Fair Value of Debt Investments Based on Maturities (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Costs	$ 184,587	$ 152,444
Due within two years, Amortized Costs	100,816	130,110
Total, Amortized Costs	285,403	282,554
Due in one year or less, Fair Value	184,334	152,212
Due within two years, Fair Value	100,378	129,466
Total, Fair Value	$ 284,712	$ 281,678