Unaudited Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement of Comprehensive Income [Abstract]
Net loss	$ (7,317)	$ (3,264)	$ (35,046)	$ (20,664)
Other comprehensive income (loss):
Unrealized income (loss) on short-term investments, net of tax	6	9	33	(3)
Gains on short-term investments reclassified into earnings, net of tax	(4)	(2)	(20)	0
Total	2	7	13	(3)
Losses (gains) on derivative instruments reclassified into earnings, net of tax	(100)	(352)	(3,985)	3,183
Losses (gains) on derivative instruments reclassified into earnings, net of tax	1,194	(774)	2,094	(1,692)
Total	1,094	(1,126)	(1,891)	1,491
Total other comprehensive income (loss)	1,096	(1,119)	(1,878)	1,488
Comprehensive loss	$ (6,221)	$ (4,383)	$ (36,924)	$ (19,176)