Investments - Cost and Fair Value of Debt Investments Based on Maturities (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Costs	$ 133,633	$ 184,587
Due within two years, Amortized Costs	77,810	100,816
Total, Amortized Costs	211,443	285,403
Due in one year or less, Fair Value	133,481	184,334
Due within two years, Fair Value	77,658	100,378
Total, Fair Value	$ 211,139	$ 284,712