CONDENSED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Treasury Stock	Accumulated Deficit	Total
BALANCE at Dec. 31, 2017	$ 10	$ 420,525		$ (360,983)	$ 59,552
BALANCE (in shares) at Dec. 31, 2017	97,203,950
Issuance of common stock for stock option exercises		5,909			5,909
Issuance of common stock for stock option exercises (in shares)	2,003,842
Issuance of common stock pursuant to the Employee Stock Purchase Plan		4,548			4,548
Issuance of common stock pursuant to the Employee Stock Purchase Plan (in shares)	1,597,778
Issuance of common stock pursuant to the Employee Stock-Settled Bonus Plans	$ 1	9,621			9,622
Issuance of common stock pursuant to the Employee Stock-Settled Bonus Plans (in shares)	1,973,386
Shares withheld for net settlement of equity awards		(5,294)			(5,294)
Shares withheld for net settlement of equity awards (in shares)	(1,072,919)
Vesting of restricted stock units (in shares)	3,714,451
Stock-based compensation		20,035			20,035
Net loss				(35,875)	(35,875)
BALANCE at Sep. 30, 2018	$ 11	455,344		(396,858)	58,497
BALANCE (in shares) at Sep. 30, 2018	105,420,488
BALANCE at Jun. 30, 2018	$ 11	442,794		(389,360)	53,445
BALANCE (in shares) at Jun. 30, 2018	102,454,185
Issuance of common stock for stock option exercises		4,500			4,500
Issuance of common stock for stock option exercises (in shares)	1,338,978
Issuance of common stock pursuant to the Employee Stock Purchase Plan		2,209			2,209
Issuance of common stock pursuant to the Employee Stock Purchase Plan (in shares)	768,596
Shares withheld for net settlement of equity awards		(1,570)			(1,570)
Shares withheld for net settlement of equity awards (in shares)	(320,355)
Vesting of restricted stock units (in shares)	1,179,084
Stock-based compensation		7,411			7,411
Net loss				(7,498)	(7,498)
BALANCE at Sep. 30, 2018	$ 11	455,344		(396,858)	58,497
BALANCE (in shares) at Sep. 30, 2018	105,420,488
BALANCE at Dec. 31, 2018	$ 11	462,004	$ (3,831)	(404,067)	54,117
BALANCE (in shares) at Dec. 31, 2018	106,206,545
Issuance of common stock for stock option exercises		5,481			$ 5,481
Issuance of common stock for stock option exercises (in shares)	1,337,433				1,337,433
Issuance of common stock pursuant to the Employee Stock Purchase Plan		4,132			$ 4,132
Issuance of common stock pursuant to the Employee Stock Purchase Plan (in shares)	1,048,302
Issuance of common stock pursuant to the Employee Stock-Settled Bonus Plans		10,485			10,485
Issuance of common stock pursuant to the Employee Stock-Settled Bonus Plans (in shares)	2,170,855
Shares withheld for net settlement of equity awards		(5,492)			(5,492)
Shares withheld for net settlement of equity awards (in shares)	(1,064,577)
Repurchase of common stock			(8,624)		$ (8,624)
Repurchase of common stock (in shares)	(1,572,030)				(1,572,030)
Vesting of restricted stock units (in shares)	3,904,145
Stock-based compensation		20,575			$ 20,575
Net loss				(40,892)	(40,892)
BALANCE at Sep. 30, 2019	$ 11	497,185	(12,455)	(444,959)	39,782
BALANCE (in shares) at Sep. 30, 2019	112,030,673
BALANCE at Jun. 30, 2019	$ 11	484,593	(10,422)	(436,756)	37,426
BALANCE (in shares) at Jun. 30, 2019	109,770,021
Issuance of common stock for stock option exercises		3,761			3,761
Issuance of common stock for stock option exercises (in shares)	780,029
Issuance of common stock pursuant to the Employee Stock Purchase Plan		2,085			2,085
Issuance of common stock pursuant to the Employee Stock Purchase Plan (in shares)	530,190
Shares withheld for net settlement of equity awards		(707)			(707)
Shares withheld for net settlement of equity awards (in shares)	(102,535)
Repurchase of common stock			(2,033)		$ (2,033)
Repurchase of common stock (in shares)	(290,844)				(290,844)
Vesting of restricted stock units (in shares)	1,343,812
Stock-based compensation		7,453			$ 7,453
Net loss				(8,203)	(8,203)
BALANCE at Sep. 30, 2019	$ 11	$ 497,185	$ (12,455)	$ (444,959)	$ 39,782
BALANCE (in shares) at Sep. 30, 2019	112,030,673