Condensed Consolidated Statements of Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Common Stock	Additional Paid-In Capital	AOCI Attributable to Parent	Accumulated Deficit
Beginning balance (in shares) at Dec. 31, 2017		44,616,586
Beginning balance at Dec. 31, 2017	$ 242,036	$ 4	$ 1,124,630	$ (314)	$ (882,284)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(180,239)				(180,239)
Unrealized gain from available-for-sale securities	(534)			(534)
Vesting of restricted shares (in shares)		24,372
Exercise of options (in shares)		460,677
Exercise of options	8,953	$ 1	8,952
Exercise of warrants (in shares)		336,059
Share-based compensation expense related to share-based awards for employee stock purchase plan	615		615
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		101,822
Issuance of common stock upon purchase by employee stock purchase plan	2,565		2,565
Share-based compensation expense	21,655		21,655
Ending balance (in shares) at Sep. 30, 2018		45,539,516
Ending balance at Sep. 30, 2018	95,051	$ 5	1,158,417	(848)	(1,062,523)
Beginning balance (in shares) at Jun. 30, 2018		45,476,455
Beginning balance at Jun. 30, 2018	136,759	$ 5	1,150,765	(1,290)	(1,012,721)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(49,802)				(49,802)
Unrealized gain from available-for-sale securities	442			442
Exercise of options (in shares)		15,929
Exercise of options	127		127
Exercise of warrants (in shares)		0
Exercise of warrants	0
Share-based compensation expense related to share-based awards for employee stock purchase plan	201		201
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		47,132
Issuance of common stock upon purchase by employee stock purchase plan	824		824
Share-based compensation expense	6,500		6,500
Ending balance (in shares) at Sep. 30, 2018		45,539,516
Ending balance at Sep. 30, 2018	95,051	$ 5	1,158,417	(848)	(1,062,523)
Beginning balance (in shares) at Dec. 31, 2018		45,563,693
Beginning balance at Dec. 31, 2018	60,632	$ 5	1,165,003	(755)	(1,103,621)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(108,278)				(108,278)
Unrealized gain from available-for-sale securities	776			776
Vesting of restricted shares (in shares)		75,331
Exercise of options (in shares)		341,337
Exercise of options	2,869		2,869
Exercise of warrants (in shares)		81,104
Exercise of warrants	1,000		1,000
Share-based compensation expense related to share-based awards for employee stock purchase plan	456		456
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		111,705
Issuance of common stock upon purchase by employee stock purchase plan	1,840		1,840
Share-based compensation expense	16,455		16,455
Ending balance (in shares) at Sep. 30, 2019		46,173,170
Ending balance at Sep. 30, 2019	(24,250)	$ 5	1,187,623	21	(1,211,899)
Beginning balance (in shares) at Jun. 30, 2019		46,125,197
Beginning balance at Jun. 30, 2019	(134)	$ 5	1,181,761	(45)	(1,181,855)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net loss	(30,044)				(30,044)
Unrealized gain from available-for-sale securities	66			66
Share-based compensation expense related to share-based awards for employee stock purchase plan	20		20
Issuance of common stock upon purchase by employee stock purchase plan (in shares)		47,973
Issuance of common stock upon purchase by employee stock purchase plan	813		813
Share-based compensation expense	5,029		5,029
Ending balance (in shares) at Sep. 30, 2019		46,173,170
Ending balance at Sep. 30, 2019	$ (24,250)	$ 5	$ 1,187,623	$ 21	$ (1,211,899)