Shelf Registration Statement (Textual) (Details) - USD ($) $ / shares in Units, shares in Thousands		2 Months Ended	12 Months Ended
	Apr. 23, 2015	Jun. 11, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 16, 2017
Stock Offering [Line Items]
Aggregate amount of common stock						$ 125,000,000
Proceeds from issuance of common stock, net					$ 9,564,000
Proceeds from sale of common stock, net of offering costs			$ 0	$ 0	9,564,000
Common Stock
Stock Offering [Line Items]
Proceeds from issuance of common stock, net					$ 183,000
Proceeds from issuance of common stock, net, shares				1,830	1,831
Cowen and Company | Equity Offering Sales Agreement under Shelf Registration Statement
Stock Offering [Line Items]
Aggregate shares of common stock	$ 10,000,000.0
Sales commission percentage	3.00%
Proceeds from issuance of common stock, net		$ 10,000,000
Proceeds from sale of common stock, net of offering costs		9,600,000
Offering costs		$ 400,000
Cowen and Company | Equity Offering Sales Agreement under Shelf Registration Statement | Weighted Average
Stock Offering [Line Items]
Average price per share		$ 5.46