Fair Value Measurements (Estimated Fair Values Of Debt Instruments) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Level 1 [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	$ 7,098	$ 3,360
Level 2 [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	23,966	3,840
Financial Liabilities Fair Value Disclosure	11,880	12,375
Marketable securities | Level 2 [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	21,491
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	7,098	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 Assets Fair Value Disclosure	200
Financial Liabilities Fair Value Disclosure	3,934	2,424
Forward Contracts [Member] | Level 2 [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	2,275	3,840
Financial Liabilities Fair Value Disclosure	3,720	1,020
Renewable Energy Program [Member] | Level 2 [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	4,226	8,931
Reported Value Measurement [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	23,966	3,840
Financial Liabilities Fair Value Disclosure	18,978	15,735
Reported Value Measurement [Member] | Marketable securities
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	21,491
Reported Value Measurement [Member] | Nymex Futures Contracts [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	7,098
Reported Value Measurement [Member] | Commodity Contract [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	200
Financial Liabilities Fair Value Disclosure	3,934	2,424
Reported Value Measurement [Member] | Forward Contracts [Member]
Debt Instrument [Line Items]
Financial Assets Fair Value Disclosure	2,275	3,840
Financial Liabilities Fair Value Disclosure	3,720	1,020
Reported Value Measurement [Member] | Renewable Energy Program [Member]
Debt Instrument [Line Items]
Financial Liabilities Fair Value Disclosure	$ 4,226	$ 8,931