Financial Instruments and Concentrations of Risk (Details) - Commodity Derivatives [Member] - Level 1 [Member] - USD ($) $ in Thousands		Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset		$ 131	$ 3,311
Derivative Asset, Fair Value, Gross Liability	[1]	(131)	(1,637)
Derivative Asset, Fair Value, Amount Not Offset Against Collateral		0	1,674
Derivative Liability, Fair Value, Gross Liability		470	1,637
Derivative Liability, Fair Value, Gross Asset	[1]	(131)	(1,637)
Derivative Liability, Fair Value, Amount Not Offset Against Collateral		$ 339	$ 0

[1] Relates primarily to exchange traded futures. Gain and loss positions on multiple contracts are settled net on a daily basis with the exchange.