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, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Commodity derivative contracts	$ 3,837
Assets, Total	3,837
Liabilities, Nonqualified employee savings plans	13,845	$ 16,158
Liabilities, Commodity derivative contracts		39,093
Business Combination, Contingent Consideration, Liability	47,730
Liabilities, Total	61,575	55,251
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, Nonqualified employee savings plans	13,845	16,158
Liabilities, Total	13,845	16,158
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, Commodity derivative contracts	3,837
Assets, Total	3,837
Liabilities, Commodity derivative contracts		39,093
Liabilities, Total		$ 39,093
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Business Combination, Contingent Consideration, Liability	47,730
Liabilities, Total	$ 47,730