Changes in Fair Value of Common Stock Warrant Liability (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Equity [Abstract]
Common stock warrant liability, beginning balance	$ 3,800	$ 1,400	$ 2,350	$ 1,403
Change in fair value of common stock warrant liability	(3,700)	(600)	(5,150)	(597)
Common stock warrant liability, ending balance	$ 7,500	$ 2,000	$ 7,500	$ 2,000