Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional paid in capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Accumulated Deficit [Member]
Balances at Dec. 31, 2017	$ 183,029	$ 3	$ 371,497	$ (715)	$ (187,756)
Balances (Shares) at Dec. 31, 2017		31,295
Adoption of ASU | ASU 2014-09 [Member]	4,803				4,803
Issuance of common stock upon public offering, net of transaction costs, values	189,097	$ 1	189,096
Issuance of common stock upon public offering, net of transaction costs, shares		3,105
Exercise of stock options	9,051		9,051
Exercise of stock options, Shares		1,267
Issuance of common stock under employee stock purchase plan	850		850
Issuance of common stock under employee stock purchase plan, shares		37
Stock-based compensation expense	11,755		11,755
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	(159)			(159)
Net income (loss)	95,631				95,631
Balances at Sep. 30, 2018	494,057	$ 4	582,249	(874)	(87,322)
Balances (Shares) at Sep. 30, 2018		35,704
Balances at Jun. 30, 2018	317,222	$ 3	386,110	(771)	(68,120)
Balances (Shares) at Jun. 30, 2018		32,275
Issuance of common stock upon public offering, net of transaction costs, values	189,097	$ 1	189,096
Issuance of common stock upon public offering, net of transaction costs, shares		3,105
Exercise of stock options	2,053		2,053
Exercise of stock options, Shares		307
Issuance of common stock under employee stock purchase plan	507		507
Issuance of common stock under employee stock purchase plan, shares		17
Stock-based compensation expense	4,483		4,483
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	(103)			(103)
Net income (loss)	(19,202)				(19,202)
Balances at Sep. 30, 2018	494,057	$ 4	582,249	(874)	(87,322)
Balances (Shares) at Sep. 30, 2018		35,704
Balances at Dec. 31, 2018	508,848	$ 4	592,580	(720)	(83,016)
Balances (Shares) at Dec. 31, 2018		36,120
Adoption of ASU | ASU 2016-02 [Member]	(33)				(33)
Adoption of ASU | ASU 2018-02 [Member]				40	(40)
Exercise of stock options	5,513		5,513
Exercise of stock options, Shares		684
Issuance of common stock under employee stock purchase plan	1,314		1,314
Issuance of common stock under employee stock purchase plan, shares		36
Stock-based compensation expense	19,979		19,979
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	1,043			1,043
Net income (loss)	(68,269)				(68,269)
Balances at Sep. 30, 2019	468,395	$ 4	619,386	363	(151,358)
Balances (Shares) at Sep. 30, 2019		36,840
Balances at Jun. 30, 2019	494,592	$ 4	610,891	471	(116,774)
Balances (Shares) at Jun. 30, 2019		36,752
Exercise of stock options	384		384
Exercise of stock options, Shares		62
Issuance of common stock under employee stock purchase plan	949		949
Issuance of common stock under employee stock purchase plan, shares		26
Stock-based compensation expense	7,162		7,162
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	(108)			(108)
Net income (loss)	(34,584)				(34,584)
Balances at Sep. 30, 2019	$ 468,395	$ 4	$ 619,386	$ 363	$ (151,358)
Balances (Shares) at Sep. 30, 2019		36,840