Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands			1 Months Ended	3 Months Ended						9 Months Ended		35 Months Ended
	Apr. 05, 2019	Nov. 30, 2016	Jan. 31, 2018	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Feb. 25, 2019
Common Stock
Net proceeds from issuance of common stock										$ 28,647	$ 55,961
Common Stock
Common Stock
Number of shares issued				3,961,643	1,180,367	1,318,481	2,026,031	1,166,535	7,475,000
Common Stock | 2016 Sales Agreement
Common Stock
Maximum aggregate proceeds from offering		$ 40,000
Number of shares issued						1,318,481						6,330,222
Net proceeds from issuance of common stock						$ 4,954						$ 38,381
Aggregate value available for issuance													$ 0
Common Stock | 2019 Sales Agreement
Common Stock
Maximum aggregate proceeds from offering	$ 50,000
Number of shares issued				3,961,643						5,142,010
Net proceeds from issuance of common stock				$ 18,618						$ 23,692
Common Stock | Follow-on Offering
Common Stock
Common stock, price per share			$ 5.00
Number of shares issued			7,475,000
Net proceeds from issuance of common stock			$ 34,704