Consolidated Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Additional paid in capital [Member]	Accumulated Other Comprehensive 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
Exercise of stock options	6,999		6,999
Exercise of stock options, Shares		961
Issuance of common stock under employee stock purchase plan	342		342
Issuance of common stock under employee stock purchase plan, shares		20
Stock-based compensation expense	7,272		7,272
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	(56)			(56)
Net income (loss)	114,833				114,833
Balances at Jun. 30, 2018	317,222	$ 3	386,110	(771)	(68,120)
Balances (Shares) at Jun. 30, 2018		32,275
Adoption of ASU | ASU 2014-09 [Member]	4,803				4,803
Balances at Mar. 31, 2018	299,340	$ 3	378,954	(903)	(78,714)
Balances (Shares) at Mar. 31, 2018		31,900
Exercise of stock options	3,174		3,174
Exercise of stock options, Shares		375
Stock-based compensation expense	3,982		3,982
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	132			132
Net income (loss)	10,594				10,594
Balances at Jun. 30, 2018	317,222	$ 3	386,110	(771)	(68,120)
Balances (Shares) at Jun. 30, 2018		32,275
Balances at Dec. 31, 2018	508,848	$ 4	592,580	(720)	(83,016)
Balances (Shares) at Dec. 31, 2018		36,120
Exercise of stock options	5,129		5,129
Exercise of stock options, Shares		622
Issuance of common stock under employee stock purchase plan	365		365
Issuance of common stock under employee stock purchase plan, shares		10
Stock-based compensation expense	12,817		12,817
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	1,151			1,151
Net income (loss)	(33,685)				(33,685)
Balances at Jun. 30, 2019	494,592	$ 4	610,891	471	(116,774)
Balances (Shares) at Jun. 30, 2019		36,752
Adoption of ASU | ASU 2016-02 [Member]	(33)				(33)
Adoption of ASU | ASU 2018-02 [Member]				40	(40)
Balances at Mar. 31, 2019	487,053	$ 4	602,425	(59)	(115,317)
Balances (Shares) at Mar. 31, 2019		36,611
Exercise of stock options	1,367		1,367
Exercise of stock options, Shares		141
Stock-based compensation expense	7,099		7,099
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	530			530
Net income (loss)	(1,457)				(1,457)
Balances at Jun. 30, 2019	$ 494,592	$ 4	$ 610,891	$ 471	$ (116,774)
Balances (Shares) at Jun. 30, 2019		36,752