Cash, Cash Equivalents, and Short-Term Investments - Summary of Cost and Estimated Fair Value of Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Short-term Investments [Abstract]
Amortized Cost of short-term investments, Due within one year	$ 28,891
Amortized Cost of short-term investments, Due within two years	12,783
Short-term investments, Amortized Cost	41,674	38,373
Estimated fair value of short-term investments, Due within one year	28,888
Estimated fair value of short-term investments, Due within two years	12,736
Short-term investments, Estimated Fair Value	$ 41,624	$ 38,381