Summary of Significant Accounting Policies (Reconciliation of Number of Shares Used in Calculation of Basic and Diluted Net Loss Per Share) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Computation of basic and diluted net loss per share:
Net loss applicable to common stockholders									$ (16,893)	$ (10,262)	$ (13,919)
Weighted-average shares of common stock outstanding									31,949	28,351	24,662
Less: weighted-average number of unvested restricted common shares outstanding											36
Weighted-average number of common shares used in calculating net loss per common share									31,949	28,351	24,626
Net loss per share attributable to common stockholders	$ (0.12)	$ (0.12)	$ (0.13)	$ (0.16)	$ (0.04)	$ (0.04)	$ (0.12)	$ (0.15)	$ (0.53)	$ (0.36)	$ (0.57)