Physical Delivery Contracts and Gas Derivatives (Details 3) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Commodity derivative assets:
Commodity derivative asset		$ 215
Other long-term assets		10
Total derivative assets		225
Commodity derivative liabilities:
Commodity derivative liability (Note 13)	6,502
Commodity derivative liability	4,299
Total derivative liabilities	10,801
Gross Recognized Assets/Liabilities [Member]
Commodity derivative assets:
Commodity derivative asset	334	624
Other long-term assets	248	250
Total derivative assets	582	874
Commodity derivative liabilities:
Commodity derivative liability (Note 13)	(6,836)	(409)
Commodity derivative liability	(4,547)	(240)
Total derivative liabilities	(11,383)	(649)
Gross Amounts Offset [Member]
Commodity derivative assets:
Commodity derivative asset	(334)	(409)
Other long-term assets	(248)	(240)
Total derivative assets	(582)	(649)
Commodity derivative liabilities:
Commodity derivative liability (Note 13)	334	409
Commodity derivative liability	248	240
Total derivative liabilities	582	649
Net Recognized Fair Value Assets/Liabilities [Member]
Commodity derivative assets:
Commodity derivative asset		215
Other long-term assets		10
Total derivative assets		225
Commodity derivative liabilities:
Commodity derivative liability (Note 13)	(6,502)
Commodity derivative liability	(4,299)
Total derivative liabilities	$ (10,801)