Changes in Fair Value of Common Stock Warrant Liability (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Warrants And Rights Note Disclosure [Abstract]
Common stock warrant liability, beginning balance	$ 8,300	$ 7,500	$ 9,300	$ 2,350
Change in fair value of common stock warrant liability	(2,400)	3,300	(3,400)	8,450
Common stock warrant liability, ending balance	$ 5,900	$ 10,800	$ 5,900	$ 10,800