Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Deficit [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Beginning balances at Dec. 31, 2015	$ 234,592	$ 28	$ 424,147	$ (189,408)	$ (175)
Beginning balances, shares at Dec. 31, 2015		28,143,573
Conversion feature of convertible senior notes due 2021, net of allocated costs	31,767		31,767
Purchase of bond hedges	(45,092)		(45,092)
Sales of warrants	33,120		33,120
Exercise of common stock options	$ 6,808	$ 1	6,807
Exercise of common stock options, shares	667,494	669,337
Issuance of common stock upon release of restricted stock units		1,384
Issuance of common stock under employee stock purchase plan	$ 3,499		3,499
Issuance of common stock under employee stock purchase plan, shares		72,568
Vesting of early exercised stock options	47		47
Stock based compensation	15,760		15,760
Tax benefit from stock option deductions	814		814
Net loss	(31,778)			(31,778)
Other comprehensive loss	(503)				(503)
Ending balances at Dec. 31, 2016	249,034	$ 29	470,869	(221,186)	(678)
Ending balances, shares at Dec. 31, 2016		28,886,862
Exercise of common stock options	$ 7,493	$ 1	7,492
Exercise of common stock options, shares	707,410	707,410
Issuance of common stock upon release of restricted stock units		83,121
Shares withheld for tax obligations	$ (991)		(991)
Shares withheld for tax obligations, shares		(13,094)
Issuance of common stock under employee stock purchase plan	4,697		4,697
Issuance of common stock under employee stock purchase plan, shares		73,262
Vesting of early exercised stock options	7		7
Stock based compensation	26,154		26,154
Net loss	(36,658)			(36,658)
Other comprehensive loss	(564)				(564)
Ending balances at Dec. 31, 2017	249,172	$ 30	508,228	(257,844)	(1,242)
Ending balances, shares at Dec. 31, 2017		29,737,561
Adoption of accounting standard	967			967
Exercise of common stock options	$ 3,976		3,976
Exercise of common stock options, shares	247,768	247,768
Issuance of common stock upon release of restricted stock units		196,274
Shares withheld for tax obligations	$ (1,742)		(1,742)
Shares withheld for tax obligations, shares		(28,107)
Issuance of common stock under employee stock purchase plan	5,520		5,520
Issuance of common stock under employee stock purchase plan, shares		110,040
Stock based compensation	36,630		36,630
Net loss	(49,205)			(49,205)
Other comprehensive loss	165				165
Ending balances at Dec. 31, 2018	$ 245,483	$ 30	$ 552,612	$ (306,082)	$ (1,077)
Ending balances, shares at Dec. 31, 2018		30,263,536