Stockholders' Equity - Common Stock (Details) - USD ($) $ in Thousands	1 Months Ended		2 Months Ended	12 Months Ended
	Nov. 30, 2017	May 31, 2016	Feb. 28, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 13, 2017
Common Stock
Maximum aggregate offering price of securities available for sale							$ 250,000
Aggregate offering price stock sale		$ 60,000
Additional offering price stock sale authorized	$ 75,000
Net proceeds from sale of common stock				$ 51,025	$ 13,946	$ 188,840
Common Stock
Common Stock
Shares Issued in Connection with Cantor Agreement (in shares)				17,722,863	3,303,800
Aggregate gross offering price available				$ 67,600
Common stock issued (in shares)			2,401,847			8,337,500
Net proceeds from sale of common stock			$ 6,100