Investments - Cost and Fair Value of Debt Investments Based on Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Costs	$ 95,682	$ 70,180
Due within two years, Amortized Costs	91,336	62,184
Total, Amortized Costs	187,018	132,364
Due in one year or less, Fair Value	95,667	70,169
Due within two years, Fair Value	91,243	61,948
Total, Fair Value	$ 186,910	$ 132,117