Net Loss Per Share - Outstanding Options, Warrants and Preferred Stock Excluded from Computation of Basic and Diluted Net Loss per Share (Detail) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Options to purchase common stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive Securities Excluded from Computation of Earnings Per Share	1,796	1,623
Convertible preferred stock (as converted) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive Securities Excluded from Computation of Earnings Per Share	8,631	7,567
Convertible preferred stock warrants (as converted) [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive Securities Excluded from Computation of Earnings Per Share	618	554
Common stock warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Anti-dilutive Securities Excluded from Computation of Earnings Per Share		2