Changes in Fair Value of Common Stock Warrant Liability (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Common Stock Warrant [Line Items]
Common stock warrant liability, beginning of period	$ 2,000	$ 5,027	$ 1,403	$ 5,700
Change in fair value of common stock warrant liability	850	(2,803)	1,447	(3,476)
Common stock warrant liability, end of period	$ 2,850	$ 2,224	$ 2,850	$ 2,224