Physical Delivery Contracts and Gas Derivatives (Details 3) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Commodity derivative assets:
Commodity derivative asset		$ 215
Other long-term assets		10
Total derivative assets		215
Commodity derivative liabilities:
Commodity derivative liability (Note 13)	4,570
Commodity derivative liability	3,447
Total derivative liabilities	8,017
Gross Recognized Assets/Liabilities [Member]
Commodity derivative assets:
Commodity derivative asset	272	624
Other long-term assets	181	250
Total derivative assets	453	874
Commodity derivative liabilities:
Commodity derivative liability (Note 13)	(4,842)	(409)
Commodity derivative liability	(3,628)	(240)
Total derivative liabilities	(8,470)	(649)
Gross Amounts Offset [Member]
Commodity derivative assets:
Commodity derivative asset	(272)	(409)
Other long-term assets	(181)	(240)
Total derivative assets	(453)	(649)
Commodity derivative liabilities:
Commodity derivative liability (Note 13)	272	409
Commodity derivative liability	181	240
Total derivative liabilities	453	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)	(4,570)
Commodity derivative liability	(3,447)
Total derivative liabilities	$ (8,017)