Summary of Significant Accounting Policies (Earnings Per Share) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Numerator [Abstract]
Net loss	$ (9,813)	$ (9,262)	$ (12,605)	$ (9,623)	$ (15,538)	$ (2,309)	$ (5,519)	$ (7,490)	$ (41,303)	$ (30,857)	$ (16,550)
Denominator [Abstract]
Weighted-average shares of common stock outstanding (shares)									38,231	36,768	35,674
Weighted-average shares of common stock used in computing net loss per share of common stock, basic and diluted (shares)									38,231	36,768	35,674
Net loss per share of common stock, basic and diluted (dollars per share)	$ (0.26)	$ (0.24)	$ (0.33)	$ (0.25)	$ (0.41)	$ (0.06)	$ (0.15)	$ (0.21)	$ (1.08)	$ (0.84)	$ (0.46)