Fair Value Measurements - Additional Information (Detail) (USD $)	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Cash and cash equivalents
Cash	$ 20,552,000	$ 15,129,000
Money market funds	62,281,000	62,278,000
Total cash and cash equivalents	82,833,000	77,407,000	69,510,000	77,376,000
Long-term investments
U.S. Treasury Bills	2,600,000	2,599,000
Total cash and cash equivalents and investments	85,433,000	80,006,000
Fair Value, Inputs, Level 1
Long-term investments
Cash and cash equivalents	82,800,000
Long-term restricted cash	$ 2,600,000