Loss Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								6 Months Ended		9 Months Ended		12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Dec. 31, 2017
Numerator:
Net loss	$ (9,898)	$ (18,819)	$ (11,805)	$ (14,204)	$ (6,129)	$ (9,201)	$ (7,747)	$ (24,535)	$ (26,009)	$ (32,282)	$ (44,828)	$ (41,483)	$ (54,726)	$ (47,612)
Less: Deemed dividend on Series B convertible preferred stock													0	(37,860)
Net loss available to common stockholders - basic and diluted													$ (54,726)	$ (85,472)
Shares used in computing net loss per share:
Weighted average basic shares outstanding (in shares)	18,638	18,625	18,594	18,483	16,774	13,659	13,697	11,018					18,585	13,787
Effect of dilutive securities (in shares)													0	0
Weighted average common shares outstanding-diluted (in shares)	22,360	18,625	18,594	18,483	16,774	13,659	13,697	11,018					18,585	13,787
Loss per common share:
Loss per share — basic (in dollars per share)	$ (0.53)	$ (1.01)	$ (0.63)	$ (0.77)	$ (2.22)	$ (0.94)	$ (0.80)	$ (2.23)					$ (2.94)	$ (6.20)
Loss per share — diluted (in dollars per share)	$ (0.84)	$ (1.01)	$ (0.63)	$ (0.77)	$ (2.22)	$ (0.94)	$ (0.80)	$ (2.23)					$ (2.94)	$ (6.20)
Antidilutive shares (in shares)													3,900	6,300