Assets and Liabilities Measured at Fair Value (Carrying Value of Assets and Liabilities Recorded at Fair Value on Recurring Basis) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Commodity derivative contracts	$ 89,358	$ 23,168
Assets, Total	89,358	23,168
Liabilities, Nonqualified employee savings plan	12,971	14,408
Liabilities, Foreign currency exchange derivative contracts	29	25
Liabilities, Total	13,000	14,433
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Nonqualified employee savings plan	12,971	14,408
Liabilities, Total	12,971	14,408
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Commodity derivative contracts	89,358	23,168
Assets, Total	89,358	23,168
Liabilities, Foreign currency exchange derivative contracts	29	25
Liabilities, Total	$ 29	$ 25