Investments - Cost and Fair Value of Debt Investments Based on Maturities (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Costs	$ 200,195	$ 184,587
Due within two years, Amortized Costs	79,570	100,816
Total, Amortized Costs	279,765	285,403
Due in one year or less, Fair Value	199,906	184,334
Due within two years, Fair Value	79,338	100,378
Total, Fair Value	$ 279,244	$ 284,712