Changes in Fair Value of Common Stock Warrant Liability (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Class Of Warrant Or Right [Line Items]
Common stock warrant liability, beginning balance	$ 7,500	$ 2,000	$ 2,350	$ 1,403
Change in fair value of common stock warrant liability	3,300	850	8,450	1,447
Common stock warrant liability, ending balance	$ 10,800	$ 2,850	$ 10,800	$ 2,850