Fair Value Measurements - Short-Term Investments (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Short-term Investments, Amortized Cost	$ 56,978	$ 38,980
Short-term Investments, Gross Unrealized Holding Gains	4	8
Short-term Investments, Gross Unrealized Holding Losses	0	0
Short-term Investments, Fair Value	56,982	38,988
U.S. Treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Short-term Investments, Amortized Cost	56,978	38,980
Short-term Investments, Gross Unrealized Holding Gains	4	8
Short-term Investments, Gross Unrealized Holding Losses	0	0
Short-term Investments, Fair Value	$ 56,982	$ 38,988