Net Loss per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Anti-dilutive securities
Net loss	$ (29,357)	$ (23,360)	$ (20,745)	$ (16,028)	$ (14,960)	$ (14,394)	$ (16,876)	$ (13,828)	$ (89,490)	$ (60,058)	$ (51,802)
Weighted average common shares outstanding, basic and diluted (in shares)									22,558	21,711	18,116
Less: weighted average unvested common shares subject to repurchase (in shares)									0	0	(5)
Weighted average common shares used in calculating net loss per share, basic and diluted (in shares)									22,558	21,711	18,111
Net loss per share, basic and diluted (in dollars per share)	$ (1.27)	$ (1.04)	$ (0.93)	$ (0.72)	$ (0.68)	$ (0.66)	$ (0.78)	$ (0.65)	$ (3.97)	$ (2.77)	$ (2.86)
Total anti-dilutive securities (in shares)									5,940	5,523	5,249
Options to purchase common stock
Anti-dilutive securities
Total anti-dilutive securities (in shares)									5,940	5,523	5,249