Shareholders' Equity (Details) - Cowen - USD ($)		3 Months Ended	12 Months Ended	18 Months Ended
	Oct. 10, 2016	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2018
Class of Stock [Line Items]
At-the-market sales agreement, maximum aggregate proceeds	$ 25,000,000.0
At-the-market sales agreement, proceeds from issuance of common stock		$ 7,200,000
Commission and issuance costs			$ 300,000
Amount of net capacity remaining on the at-the-market sales agreement		$ 16,700,000		$ 16,700,000
Common Class A
Class of Stock [Line Items]
At-the-market sales agreement, shares issued (in shares)		0	1,983,023	2,340,879
Maximum
Class of Stock [Line Items]
Percentage of gross proceeds payable as commission	3.00%