Net Loss per Share - Reconciliation of Number of Shares Used in Calculation of Basic and Diluted Net Loss Per Share (Detail)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Computation of basic and diluted net loss per share:
Weighted-average shares of common stock outstanding	28,024,000	15,901,000
Less: weighted-average number of unvested restricted common shares outstanding		58,000
Weighted-average number of common shares used in calculating net loss per common share	28,023,708	15,842,743