Investments - Cost and Fair Value of Debt Investments Based on Maturities (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Costs	$ 31,348	$ 184,587
Due within two years, Amortized Costs	40,032	100,816
Total, Amortized Costs	71,380	285,403
Due in one year or less, Fair Value	31,254	184,334
Due within two years, Fair Value	39,763	100,378
Total, Fair Value	$ 71,017	$ 284,712