Assets and Liabilities Measured at Fair Value (Carrying Value of Assets and Liabilities Recorded at Fair Value on Recurring Basis) (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Nonqualified employee savings plans	$ 16,158	$ 13,904
Liabilities, Commodity derivative contracts	39,093	48,864
Liabilities, Foreign currency exchange derivative contracts		73
Liabilities, Total	55,251	62,841
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Nonqualified employee savings plans	16,158	13,904
Liabilities, Total	16,158	13,904
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Commodity derivative contracts	39,093	48,864
Liabilities, Foreign currency exchange derivative contracts		73
Liabilities, Total	$ 39,093	$ 48,937