Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Short-term investments	$ 104,350	$ 26,816
Interest rate swap agreements	82	52
Total assets	248,275	106,124
Liabilities
Accrued other liabilities	1,331	5,149
Total liabilities	14,690	177,018
Money market funds
Assets
Cash equivalents:	143,843	79,256
Natural gas fixed price forward contracts
Liabilities
Derivatives:	9,729	15,368
Embedded derivative on 6% promissory notes
Liabilities
Derivatives:		140,771
Interest rate swap agreements
Liabilities
Derivatives:	3,630	5,905
Preferred stock warrants
Liabilities
Stock warrants:		9,825
Level 1
Assets
Short-term investments	104,350	26,816
Interest rate swap agreements	0	0
Total assets	248,193	106,072
Liabilities
Accrued other liabilities	1,331	0
Total liabilities	1,331	0
Level 1 | Money market funds
Assets
Cash equivalents:	143,843	79,256
Level 1 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	0	0
Level 1 | Embedded derivative on 6% promissory notes
Liabilities
Derivatives:		0
Level 1 | Interest rate swap agreements
Liabilities
Derivatives:	0	0
Level 1 | Preferred stock warrants
Liabilities
Stock warrants:		0
Level 2
Assets
Short-term investments	0	0
Interest rate swap agreements	82	52
Total assets	82	52
Liabilities
Accrued other liabilities	0	5,149
Total liabilities	3,630	11,054
Level 2 | Money market funds
Assets
Cash equivalents:	0	0
Level 2 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	0	0
Level 2 | Embedded derivative on 6% promissory notes
Liabilities
Derivatives:		0
Level 2 | Interest rate swap agreements
Liabilities
Derivatives:	3,630	5,905
Level 2 | Preferred stock warrants
Liabilities
Stock warrants:		0
Level 3
Assets
Short-term investments	0	0
Interest rate swap agreements	0	0
Total assets	0	0
Liabilities
Accrued other liabilities	0	0
Total liabilities	9,729	165,964
Level 3 | Money market funds
Assets
Cash equivalents:	0	0
Level 3 | Natural gas fixed price forward contracts
Liabilities
Derivatives:	9,729	15,368
Level 3 | Embedded derivative on 6% promissory notes
Liabilities
Derivatives:		140,771
Level 3 | Interest rate swap agreements
Liabilities
Derivatives:	0	0
Level 3 | Preferred stock warrants
Liabilities
Stock warrants:	$ 0	$ 9,825