Related Parties - Issuance of Series A Preferred Stock (Details)					1 Months Ended	9 Months Ended
	Jul. 13, 2017	Apr. 17, 2017	Jan. 19, 2017	Aug. 25, 2016 USD ($) shares	Aug. 31, 2016 director	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)
Series A Perpetual Convertible Preferred Stock
Proceeds from sale of preferred stock						$ 88,236,000
Anticipated aggregate maximum offering price							$ 50,000,000
Anticipated aggregate maximum offering price for S3 registration							$ 25,000,000
Low | Metalmark | Beneficial owner
Series A Perpetual Convertible Preferred Stock
Beneficial ownership interest (as a percent)				5.00%
Series A preferred stock
Series A Perpetual Convertible Preferred Stock
Dividend rate (as a percent)	8.00%	8.00%	8.00%	8.00%
Shares issued (in shares) | shares				1,840,000
Proceeds from sale of preferred stock				$ 88,300,000
Series A preferred stock | Director
Series A Perpetual Convertible Preferred Stock
Number of directors who are managing directors of the related party | director					2
Shares issued (in shares) | shares				200,000
Proceeds from sale of preferred stock				$ 10,000,000
Underwriting discounts and commissions				$ 400,000