Stockholders' Equity (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	May 31, 2016
Stockholders' Equity
Net proceeds from sale of common stock	$ 21,489	$ 2,551
Cantor agreement
Stockholders' Equity
Common stock issued (in shares)	6,731,066
Net proceeds from sale of common stock	$ 21,500
Aggregate gross offering price available	$ 23,100
Cantor agreement | Maximum
Stockholders' Equity
Aggregate offering price of common stock			$ 60,000