Fair Value - Change in Level 3 Financial Assets (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance	$ 9,729	$ 165,964
Settlement of natural gas fixed price forward contracts	(2,741)	(2,871)
Embedded derivative on notes		5,533
Changes in fair value	1,477	20,909
Reclassification of preferred stock warrants liability to common stock warrants and derivative liability into additional paid-in-capital		(178,090)
Balance	8,465	11,445
Natural gas fixed price forward contract
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance	9,729	15,368
Settlement of natural gas fixed price forward contracts	(2,741)	(2,871)
Embedded derivative on notes		0
Changes in fair value	1,477	(1,052)
Reclassification of preferred stock warrants liability to common stock warrants and derivative liability into additional paid-in-capital		0
Balance	8,465	11,445
Preferred stock warrants
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance	0	9,825
Settlement of natural gas fixed price forward contracts		0
Embedded derivative on notes		0
Changes in fair value	0	(8,943)
Reclassification of preferred stock warrants liability to common stock warrants and derivative liability into additional paid-in-capital		(882)
Balance	0	0
Embedded Derivative Liability
Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance	0	140,771
Settlement of natural gas fixed price forward contracts		0
Embedded derivative on notes		5,533
Changes in fair value	0	30,904
Reclassification of preferred stock warrants liability to common stock warrants and derivative liability into additional paid-in-capital		(177,208)
Balance	$ 0	$ 0