Assets and Liabilities Measured at Fair Value (Fair Value Measurements for Assets and Liabilities) (Details) (Fair Value, Measurements, Recurring [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Commodity derivative contracts	$ 23,168	$ 1,970
Liabilities, Nonqualified employee savings plan	(14,408)	(13,267)
Liabilities, Foreign currency exchange derivative contracts	(25)	(1,038)
Liabilities, Total	(14,433)	(14,305)
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Commodity derivative contracts
Liabilities, Nonqualified employee savings plan	(14,408)	(13,267)
Liabilities, Foreign currency exchange derivative contracts
Liabilities, Total	(14,408)	(13,267)
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Commodity derivative contracts	23,168	1,970
Liabilities, Nonqualified employee savings plan
Liabilities, Foreign currency exchange derivative contracts	(25)	(1,038)
Liabilities, Total	(25)	(1,038)
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring Basis [Line Items]
Assets, Commodity derivative contracts
Liabilities, Nonqualified employee savings plan
Liabilities, Foreign currency exchange derivative contracts
Liabilities, Total