Financial Instruments and Concentrations of Risk (Details) - Not Designated as Hedging Instrument [Member] - Fair Value, Measurements, Recurring [Member] - Commodity Contract [Member] - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset	$ 0	$ 0
Derivative Liability	1,328	339
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	68	131
Derivative Asset, Fair Value, Gross Liability	(68)	(131)
Derivative Liability, Fair Value, Gross Liability	1,396	470
Derivative Liability, Fair Value, Gross Asset	$ (68)	$ (131)