Quarterly Financial Data (unaudited) - Summary of Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Data [Abstract]
Revenues	$ 12,400	$ 9,188	$ 6,639	$ 5,331	$ 2,954	$ 3,077	$ 3,001	$ 2,410		$ 11,442	$ 41,971
Total operating expenses	30,281	26,867	26,130	23,931	13,700	20,245	18,771	17,393	$ 107,209	70,109	52,504
Net loss	$ (15,676)	$ (15,789)	$ (18,413)	$ (17,820)	$ (10,066)	$ (16,721)	$ (15,461)	$ (14,762)	$ (67,698)	$ (57,010)	$ (10,380)
Net loss per common share, basic and diluted	$ (0.39)	$ (0.39)	$ (0.49)	$ (0.50)	$ (0.28)	$ (0.50)	$ (0.50)	$ (0.47)	$ (1.76)	$ (1.74)	$ (0.38)
Weighted average number of shares, basic and diluted	40,259,575	40,116,644	37,440,024	35,827,235	35,684,201	33,525,567	31,200,773	31,089,310	38,386,906	32,832,514	27,475,792