Fair Value - Change in Level 3 Financial Assets (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance	$ 165,964	$ 147,277
Settlement of natural gas fixed price forward contracts	(2,871)	(4,248)
Embedded derivative on notes	5,533	6,804
Changes in fair value	20,909	16,131
Conversion of preferred stock warrants liability and derivative liability to common shares	(178,090)
Balance	11,445	165,964
Natural gas fixed price forward contract
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance	15,368	18,585
Settlement of natural gas fixed price forward contracts	(2,871)	(4,248)
Changes in fair value	(1,052)	1,031
Conversion of preferred stock warrants liability and derivative liability to common shares	0
Balance	11,445	15,368
Preferred stock warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance	9,825	12,885
Changes in fair value	(8,943)	(3,060)
Conversion of preferred stock warrants liability and derivative liability to common shares	(882)
Balance	0	9,825
Derivative liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance	140,771	115,807
Embedded derivative on notes	5,533	6,804
Changes in fair value	30,904	18,160
Conversion of preferred stock warrants liability and derivative liability to common shares	(177,208)
Balance	$ 0	$ 140,771