Investments - Cost and Fair Value of Debt Investments Based on Maturities (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Costs	$ 175,910	$ 184,587
Due within two years, Amortized Costs	56,171	100,816
Total, Amortized Costs	232,081	285,403
Due in one year or less, Fair Value	175,682	184,334
Due within two years, Fair Value	56,053	100,378
Total, Fair Value	$ 231,735	$ 284,712