Related Parties - Issuance of Series A Preferred Stock (Details)			1 Months Ended
	Jan. 19, 2017	Aug. 25, 2016 USD ($) shares	Aug. 31, 2016 director
Minimum | 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%
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