Stockholders' Equity - Sales Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	5 Months Ended	12 Months Ended
	Jul. 31, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity [Abstract]
Stock issued (shares)	3,243,882	10,314,250			10,890,915
Proceeds from sale of common stock under an at-the-market sales agreement (Note 7)	$ 23,745	$ 104,527	$ 0	$ 128,272	$ 73,869