Fair Value Measurements (Estimated Fair Values Of Debt Instruments) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Level 1 [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	$ 1,309	$ 3,360
Level 2 [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	16,708	3,840
Financial Liabilities Fair Value Disclosure	10,335	12,375
Marketable securities | Level 2 [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	16,197
Nymex Futures Contracts [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure		3,360
Nymex Futures Contracts [Member] | Level 1 [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	1,309	3,360
Nymex Futures Contracts [Member] | Level 2 [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	0	0
Commodity Contract [Member] | Level 2 [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	3,580	2,424
Forward Contracts [Member] | Level 2 [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	511	3,840
Financial Liabilities Fair Value Disclosure	3,730	1,020
Renewable Energy Program [Member] | Level 2 [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	3,025	8,931
Reported Value Measurement [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	16,708	3,840
Financial Liabilities Fair Value Disclosure	11,644	15,735
Reported Value Measurement [Member] | Marketable securities
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	16,197
Reported Value Measurement [Member] | Nymex Futures Contracts [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	1,309
Reported Value Measurement [Member] | Commodity Contract [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	3,580	2,424
Reported Value Measurement [Member] | Forward Contracts [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	511	3,840
Financial Liabilities Fair Value Disclosure	3,730	1,020
Reported Value Measurement [Member] | Renewable Energy Program [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	$ 3,025	$ 8,931