CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income	Accumulated Earnings (Deficit)	Total
Balances at Dec. 31, 2014	$ 239,961	$ 76,809	$ (37)	$ 47,714	$ 364,447
Balances (in shares) at Dec. 31, 2014	59,293,428
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options	$ 7,860				7,860
Issuance of common stock upon exercise of options (in shares)	1,137,303
Issuance of common stock under employee stock purchase plan	$ 2,462				2,462
Issuance of common stock under employee stock purchase plan (in shares)	164,674
Issuance of common stock in conjunction with vesting of restricted stock units (in shares)	191,904
Stock-based compensation	$ 14,228				14,228
Shares withheld for payment of employee's withholding tax liability		(2,812)			(2,812)
Windfall tax benefit		4,625			4,625
Net loss				(75,738)	(75,738)
Unrealized gain (loss) on available-for-sale securities, net of tax			(17)		(17)
Balances at Dec. 31, 2015	$ 264,511	78,622	(54)	(28,024)	315,055
Balances (in shares) at Dec. 31, 2015	60,787,309
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options	$ 6,693				$ 6,693
Issuance of common stock upon exercise of options (in shares)	715,655				711,863
Issuance of common stock under employee stock purchase plan	$ 3,258				$ 3,258
Issuance of common stock under employee stock purchase plan (in shares)	201,264
Issuance of common stock in conjunction with vesting of restricted stock units (in shares)	261,960
Stock-based compensation	$ 17,172				17,172
Shares withheld for payment of employee's withholding tax liability		(3,342)			(3,342)
Windfall tax benefit		637			637
Net loss				(88,720)	(88,720)
Unrealized gain (loss) on available-for-sale securities, net of tax			35		35
Balances at Dec. 31, 2016	$ 291,634	75,917	(19)	(116,744)	$ 250,788
Balances (in shares) at Dec. 31, 2016	61,966,188				61,966,188
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon exercise of options	$ 6,979				$ 6,979
Issuance of common stock upon exercise of options (in shares)	1,001,142				1,000,892
Issuance of common stock under employee stock purchase plan	$ 1,960				$ 1,960
Issuance of common stock under employee stock purchase plan (in shares)	261,569
Issuance of common stock in conjunction with vesting of restricted stock units (in shares)	171,449
Stock-based compensation	$ 13,016				13,016
Shares withheld for payment of employee's withholding tax liability		(753)			(753)
Net loss				(102,496)	(102,496)
Unrealized gain (loss) on available-for-sale securities, net of tax			14		14
Balances at Dec. 31, 2017	$ 313,857	$ 75,164	$ (5)	(219,508)	$ 169,508
Balances (in shares) at Dec. 31, 2017	63,400,348				63,400,348
Increase (Decrease) in Stockholders' Equity
Cumulative effect adjustment from adoption of ASU | ASU 2016-09	$ 268			$ (268)