Investments - Cost and Fair Value of Debt Investments Based on Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Costs	$ 164,123	$ 152,444
Due within two years, Amortized Costs	120,780	130,110
Total, Amortized Costs	284,903	282,554
Due in one year or less, Fair Value	164,071	152,212
Due within two years, Fair Value	120,956	129,466
Total, Fair Value	$ 285,027	$ 281,678