Financial Instruments and Concentrations of Risk (Details) (Commodity Derivatives [Member], Level 1 [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Commodity Derivatives [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 3,311		$ 36
Derivative Asset, Fair Value, Gross Liability	(1,637)	[1]	(36)	[1]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral	1,674		0
Derivative Liability, Fair Value, Gross Liability	1,637		96
Derivative Liability, Fair Value, Gross Asset	(1,637)	[1]	(36)	[1]
Derivative Liability, Fair Value, Amount Not Offset Against Collateral	$ 0		$ 60

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