Financial Instruments - Fair value of commodity derivative assets and liabilities (Details) - Commodity Contract [Member] - Not Designated as Hedging Instrument [Member] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Other Current Assets [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset	$ 445	$ 1,674
Other Current Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability	$ 0	$ 0