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
Adoption of ASU | ASU 2014-09 [Member]	4,803				4,803
Exercise of stock options	3,824		3,824
Exercise of stock options, Shares		586
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	3,291		3,291
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	(188)			(188)
Net income (loss)	104,239				104,239
Balances at Mar. 31, 2018	299,340	$ 3	378,954	(903)	(78,714)
Balances (Shares) at Mar. 31, 2018		31,900
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	3,762		3,762
Exercise of stock options, Shares		481
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	5,718		5,718
Unrealized gain (loss) on available-for-sale securities, net of reclassifications and income tax expense	621			621
Net income (loss)	(32,228)				(32,228)
Balances at Mar. 31, 2019	$ 487,053	$ 4	$ 602,425	$ (59)	$ (115,317)
Balances (Shares) at Mar. 31, 2019		36,611