Investments and Fair Value Measurements - Amortized Cost and Estimated Fair Value of Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investments Debt And Equity Securities [Abstract]
Mature in less than one year, Amortized cost	$ 11,829
Mature in one to three years, Amortized cost	51,668
Total short-term investments, Amortized cost	63,497	80,664
Mature in less than one year, Fair value	11,855
Mature in one to three years, Fair value	51,695
Fair value	$ 63,550	$ 80,966