Condensed Statements of Stockholders' Equity (Unaudited) - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock	Additional Paid- in- Capital	Accumulated Other Comprehensive Income (Loss)	Accumulated Deficit
Balances at Dec. 31, 2017	$ (7,615)	$ 1	$ 21,773		$ (29,389)
Balance (in shares) at Dec. 31, 2017		7,888
Common stock issued	1,401		1,401
Common stock issued (in shares)		56
Net loss	(2,693)				(2,693)
Balances at Mar. 31, 2018	(8,907)	$ 1	23,174		(32,082)
Balance (in shares) at Mar. 31, 2018		7,944
Balances at Dec. 31, 2017	(7,615)	$ 1	21,773		(29,389)
Balance (in shares) at Dec. 31, 2017		7,888
Settlement of stock-based compensation liability	18,243
Forfeitures of liability-classified awards	1,313
Net loss	(15,895)
Balances at Sep. 30, 2018	6,481	$ 1	51,764		(45,284)
Balance (in shares) at Sep. 30, 2018		8,204
Balances at Mar. 31, 2018	(8,907)	$ 1	23,174		(32,082)
Balance (in shares) at Mar. 31, 2018		7,944
Common stock issued	6,500		6,500
Common stock issued (in shares)		260
Settlement of stock-based compensation liability	18,243		18,243
Forfeitures of liability-classified awards	1,313		1,313
Stock-based compensation	1,024		1,024
Net loss	(8,395)				(8,395)
Balances at Jun. 30, 2018	9,778	$ 1	50,254		(40,477)
Balance (in shares) at Jun. 30, 2018		8,204
Stock-based compensation	1,510		1,510
Net loss	(4,807)				(4,807)
Balances at Sep. 30, 2018	6,481	$ 1	51,764		(45,284)
Balance (in shares) at Sep. 30, 2018		8,204
Balances at Dec. 31, 2018	66,705	$ 1	126,925		(60,221)
Balance (in shares) at Dec. 31, 2018		12,689
Adoption of accounting standard (See Note 2) at Dec. 31, 2018	21				21
Balances at Dec. 31, 2018	66,726				(60,200)
Common stock issued (in shares)		148
Stock-based compensation	7,745		7,745
Net loss	(14,674)				(14,674)
Balances at Mar. 31, 2019	59,797	$ 1	134,670		(74,874)
Balance (in shares) at Mar. 31, 2019		12,837
Balances at Dec. 31, 2018	66,705	$ 1	126,925		(60,221)
Balance (in shares) at Dec. 31, 2018		12,689
Adoption of accounting standard (See Note 2) at Dec. 31, 2018	21				21
Balances at Dec. 31, 2018	66,726				(60,200)
Other comprehensive income (loss)	38
Net loss	(47,214)
Balances at Sep. 30, 2019	41,741	$ 1	149,116	$ 38	(107,414)
Balance (in shares) at Sep. 30, 2019		13,408
Balances at Mar. 31, 2019	59,797	$ 1	134,670		(74,874)
Balance (in shares) at Mar. 31, 2019		12,837
Common stock issued	37		37
Common stock issued (in shares)		384
Stock-based compensation	7,132		7,132
Other comprehensive income (loss)	51			51
Net loss	(15,122)				(15,122)
Balances at Jun. 30, 2019	51,895	$ 1	141,839	51	(89,996)
Balance (in shares) at Jun. 30, 2019		13,221
Common stock issued		$ 187
Stock-based compensation	7,277		7,277
Other comprehensive income (loss)	(13)			(13)
Net loss	(17,418)				(17,418)
Balances at Sep. 30, 2019	$ 41,741	$ 1	$ 149,116	$ 38	$ (107,414)
Balance (in shares) at Sep. 30, 2019		13,408