Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock	APIC	Accumulated deficit
Beginning balance (in shares) at Dec. 31, 2014		79,811,769
Beginning balance at Dec. 31, 2014	$ 249,198	$ 8	$ 460,179	$ (210,989)
Increase (Decrease) in Stockholders' Equity
Net loss	(64,911)			(64,911)
Stock-based compensation	43,888		43,888
Repurchase of common stock awards	(100)		(100)
Issuance of warrants and change in fair value of unvested warrants related to marketing agreements	(803)		(803)
Net exercise of warrants to purchase common stock (in shares)		959,676
Net exercise of warrants to purchase common stock	0	$ 0	0
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes (in shares)		2,245,290
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes	5,420		5,420
Ending balance at Dec. 31, 2015	232,692	$ 8	508,584	(275,900)
Ending balance (in shares) at Dec. 31, 2015		83,016,735
Increase (Decrease) in Stockholders' Equity
Cumulative-effect of accounting change adopted as of January 1, 2016			627	(627)
Net loss	(41,708)			(41,708)
Stock-based compensation	25,751		25,751
Issuance of warrants and change in fair value of unvested warrants related to marketing agreements	46		46
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes (in shares)		3,142,792
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes	7,800	$ 1	7,799
Ending balance at Dec. 31, 2016	$ 224,581	$ 9	542,807	(318,235)
Ending balance (in shares) at Dec. 31, 2016	86,159,527	86,159,527
Increase (Decrease) in Stockholders' Equity
Net loss	$ (32,849)			(32,849)
Issuance of common stock in follow-on offering, net of underwriting discounts and offering costs (in shares)		1,150,000
Issuance of common stock in follow-on offering, net of underwriting discounts and offering costs	17,398		17,398
Stock-based compensation	33,648		33,648
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes (in shares)		13,119,129
Shares issued in connection with employee stock plans, net of shares withheld for employee taxes	70,340	$ 1	70,339
Ending balance at Dec. 31, 2017	$ 313,118	$ 10	$ 664,192	$ (351,084)
Ending balance (in shares) at Dec. 31, 2017	100,428,656	100,428,656