Investments - Cost and Fair Value of Debt Investments Based on Maturities (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Available-for-sale Securities, Debt Maturities [Abstract]
Due in one year or less, Amortized Costs	$ 70,180	$ 32,536
Due within two years, Amortized Costs	62,184	49,558
Total, Amortized Costs	132,364	82,094
Due in one year or less, Fair Value	70,169	32,538
Due within two years, Fair Value	61,948	49,593
Total, Fair Value	$ 132,117	$ 82,131