Investments - Cost and Fair Value of Debt Investments Based on Maturities (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Costs	$ 171,336	$ 152,444
Due within two years, Amortized Costs	113,995	130,110
Total, Amortized Costs	285,331	282,554
Due in one year or less, Fair Value	171,152	152,212
Due within two years, Fair Value	113,784	129,466
Total, Fair Value	$ 284,936	$ 281,678