Statements of Stockholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Balances at Dec. 31, 2017	$ 324,515	$ 138	$ 927,420	$ (602,654)	$ (389)
Balances (in shares) at Dec. 31, 2017		137,894
Net loss	(20,588)			(20,588)
Other comprehensive income (loss)	(296)				(296)
Stock compensation	2,335		2,335
Issuance of common stock upon exercise of stock options	1,304		1,304
Issuance of common stock upon exercise of stock options (in shares)		446
Balances at Mar. 31, 2018	307,270	$ 138	931,059	(623,242)	(685)
Balances (in shares) at Mar. 31, 2018		138,340
Balances at Dec. 31, 2017	324,515	$ 138	927,420	(602,654)	(389)
Balances (in shares) at Dec. 31, 2017		137,894
Net loss	(53,671)
Balances at Sep. 30, 2018	280,506	$ 139	936,900	(656,325)	(208)
Balances (in shares) at Sep. 30, 2018		138,589
Balances at Mar. 31, 2018	307,270	$ 138	931,059	(623,242)	(685)
Balances (in shares) at Mar. 31, 2018		138,340
Net loss	(17,199)			(17,199)
Other comprehensive income (loss)	266				266
Stock compensation	3,550		3,550
Issuance of common stock upon exercise of stock options	570	$ 1	569
Issuance of common stock upon exercise of stock options (in shares)		206
Issuance of common stock under the employee stockpurchase plan	102		102
Issuance of common stock under the employee stock purchase plan (in shares)		40
Balances at Jun. 30, 2018	294,559	$ 139	935,280	(640,441)	(419)
Balances (in shares) at Jun. 30, 2018		138,586
Net loss	(15,884)			(15,884)
Other comprehensive income (loss)	211				211
Stock compensation	1,611		1,611
Issuance of common stock upon exercise of stock options	9		9
Issuance of common stock upon exercise of stock options (in shares)		3
Balances at Sep. 30, 2018	280,506	$ 139	936,900	(656,325)	(208)
Balances (in shares) at Sep. 30, 2018		138,589
Balances at Dec. 31, 2018	266,012	$ 139	938,998	(672,926)	(199)
Balances (in shares) at Dec. 31, 2018		138,716
Net loss	(18,959)			(18,959)
Other comprehensive income (loss)	320				320
Stock compensation	1,566		1,566
Balances at Mar. 31, 2019	248,939	$ 139	940,564	(691,885)	121
Balances (in shares) at Mar. 31, 2019		138,716
Balances at Dec. 31, 2018	266,012	$ 139	938,998	(672,926)	(199)
Balances (in shares) at Dec. 31, 2018		138,716
Net loss	$ (57,968)
Issuance of common stock upon exercise of stock options (in shares)	1,142
Balances at Sep. 30, 2019	$ 217,216	$ 140	947,747	(730,894)	223
Balances (in shares) at Sep. 30, 2019		139,891
Balances at Mar. 31, 2019	248,939	$ 139	940,564	(691,885)	121
Balances (in shares) at Mar. 31, 2019		138,716
Net loss	(19,426)			(19,426)
Other comprehensive income (loss)	172				172
Stock compensation	1,588		1,588
Issuance of common stock upon exercise of stock options	291		291
Issuance of common stock upon exercise of stock options (in shares)		103
Issuance of common stock under the employee stockpurchase plan	79		79
Issuance of common stock under the employee stock purchase plan (in shares)		32
Balances at Jun. 30, 2019	231,643	$ 139	942,522	(711,311)	293
Balances (in shares) at Jun. 30, 2019		138,851
Net loss	(19,583)			(19,583)
Other comprehensive income (loss)	(70)				(70)
Stock compensation	1,707		1,707
Issuance of common stock upon exercise of stock options	3,519	$ 1	3,518
Issuance of common stock upon exercise of stock options (in shares)		1,040
Balances at Sep. 30, 2019	$ 217,216	$ 140	$ 947,747	$ (730,894)	$ 223
Balances (in shares) at Sep. 30, 2019		139,891