Condensed Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Public Offering	At-the-Market Offering	Common Stock	Common Stock Public Offering	Common Stock At-the-Market Offering	Additional Paid-In Capital	Additional Paid-In Capital Public Offering	Additional Paid-In Capital At-the-Market Offering	Accumulated Deficit	Accumulated Other Comprehensive (Loss) Income
Balance at Dec. 31, 2017	$ 150,307			$ 4			$ 300,906			$ (150,333)	$ (270)
Balance, shares at Dec. 31, 2017				35,759,000
Exercise of stock options	247						247
Exercise of stock options, shares				125,000
Issuance of common stock per employee stock plan purchases	370						370
Issuance of common stock per employee stock plan purchases, shares				118,000
Issuance of stock			$ 5,611						$ 5,611
Issuance of stock, shares						1,202,000
Stock-based compensation expense	5,617						5,617
Cumulative-effect adjustment from adoption of accounting standard | ASC 606	8,792									8,792
Net loss	(35,130)									(35,130)
Unrealized gain on available-for-sale securities	96										96
Balance at Sep. 30, 2018	135,910			$ 4			312,751			(176,671)	(174)
Balance, shares at Sep. 30, 2018				37,204,000
Balance at Jun. 30, 2018	147,166			$ 4			305,235			(157,822)	(251)
Balance, shares at Jun. 30, 2018				35,963,000
Exercise of stock options	89						89
Exercise of stock options, shares				40,000
Issuance of stock			5,611						5,611
Issuance of stock, shares						1,201,000
Stock-based compensation expense	1,816						1,816
Net loss	(18,849)									(18,849)
Unrealized gain on available-for-sale securities	77										77
Balance at Sep. 30, 2018	135,910			$ 4			312,751			(176,671)	(174)
Balance, shares at Sep. 30, 2018				37,204,000
Balance at Dec. 31, 2018	126,714			$ 4			322,993			(196,170)	(113)
Balance, shares at Dec. 31, 2018				38,834,000
Exercise of stock options	$ 229						229
Exercise of stock options, shares	82,000			82,000
Issuance of common stock per employee stock plan purchases	$ 378						378
Issuance of common stock per employee stock plan purchases, shares				91,000
Issuance of stock		$ 53,761	$ 2,523		$ 1			$ 53,760	$ 2,523
Issuance of stock, shares					14,375,000	393,000
Stock-based compensation expense	5,008						5,008
Cumulative-effect adjustment from adoption of accounting standard | ASC 2018-07							71			(71)
Net loss	(68,132)									(68,132)
Unrealized gain on available-for-sale securities	178										178
Balance at Sep. 30, 2019	120,659			$ 5			384,962			(264,373)	65
Balance, shares at Sep. 30, 2019				53,775,000
Balance at Jun. 30, 2019	139,382			$ 5			383,419			(244,080)	38
Balance, shares at Jun. 30, 2019				53,771,000
Exercise of stock options	13						13
Exercise of stock options, shares				4,000
Stock-based compensation expense	1,530						1,530
Net loss	(20,293)									(20,293)
Unrealized gain on available-for-sale securities	27										27
Balance at Sep. 30, 2019	$ 120,659			$ 5			$ 384,962			$ (264,373)	$ 65
Balance, shares at Sep. 30, 2019				53,775,000