Fair Value - Financial Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Assets
Short-term investments	$ 15,703	$ 26,816
Bank loan swap agreements	912	52
Total assets	107,995	106,124
Liabilities
Total liabilities	194,929	171,868
Money market funds
Assets
Cash equivalents	91,380	79,256
Natural gas fixed price forward contracts
Liabilities
Derivatives	13,127	15,368
Embedded derivative on 6% promissory notes
Liabilities
Derivatives	176,686	140,771
Bank loan swap agreements
Liabilities
Derivatives	2,747	5,904
Preferred stock warrants
Liabilities
Stock warrants	2,369	9,825
Level 1
Assets
Short-term investments	15,703	26,816
Bank loan swap agreements	0	0
Total assets	107,083	106,072
Liabilities
Total liabilities	0	0
Level 1 | Money market funds
Assets
Cash equivalents	91,380	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	0
Level 1 | Bank loan swap agreements
Liabilities
Derivatives	0	0
Level 1 | Preferred stock warrants
Liabilities
Stock warrants	0	0
Level 2
Assets
Short-term investments	0	0
Bank loan swap agreements	912	52
Total assets	912	52
Liabilities
Total liabilities	2,747	5,904
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	0
Level 2 | Bank loan swap agreements
Liabilities
Derivatives	2,747	5,904
Level 2 | Preferred stock warrants
Liabilities
Stock warrants	0	0
Level 3
Assets
Short-term investments	0	0
Bank loan swap agreements	0	0
Total assets	0	0
Liabilities
Total liabilities	192,182	165,964
Level 3 | Money market funds
Assets
Cash equivalents	0	0
Level 3 | Natural gas fixed price forward contracts
Liabilities
Derivatives	13,127	15,368
Level 3 | Embedded derivative on 6% promissory notes
Liabilities
Derivatives	176,686	140,771
Level 3 | Bank loan swap agreements
Liabilities
Derivatives	0	0
Level 3 | Preferred stock warrants
Liabilities
Stock warrants	$ 2,369	$ 9,825