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Fair Value (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Asset	$ 36,591	$ 46,849
Liability	86,732	102,453
Cash and cash equivalents	14,861	14,656	$ 26,273	$ 16,172
Fixed Income Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	500	500
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets	(33,535)	(36,453)
Assets, Fair Value Disclosure	11,270	18,298
Counterparty and collateral netting, liabilities	(71,312)	(88,200)
Financial Liabilities Fair Value Disclosure	15,420	14,253
Fair Value, Measurements, Recurring [Member] \| Natural Gas Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets	(123)	(31)
Derivative Asset	0	0
Counterparty and collateral netting, liabilities	(123)	(31)
Derivative Liability	2,104	2,774
Fair Value, Measurements, Recurring [Member] \| Power Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, liabilities	0	0
Derivative Liability	612	2,488
Fair Value, Measurements, Recurring [Member] \| Interest Rate Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets	(2,284)	(440)
Derivative Asset	2,806	10,126
Counterparty and collateral netting, liabilities	(5,144)	(970)
Derivative Liability	10,066	6,861
Fair Value, Measurements, Recurring [Member] \| Energy commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets	(31,113)	(35,982)
Derivative Asset	250	270
Counterparty and collateral netting, liabilities	(66,030)	(87,199)
Derivative Liability	2,638	2,084
Fair Value, Measurements, Recurring [Member] \| Power Option Agreement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, liabilities		0
Derivative Liability		1
Fair Value, Measurements, Recurring [Member] \| Foreign Exchange Contract [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets	(15)
Derivative Asset	0
Counterparty and collateral netting, liabilities	(15)	0
Derivative Liability	0	45
Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value Disclosure	8,214	7,902
Financial Liabilities Fair Value Disclosure	0	0
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value Disclosure	36,468	46,818
Financial Liabilities Fair Value Disclosure	83,893	97,159
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Interest Rate Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Asset	5,090	10,566
Liability	15,210	7,831
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Energy commodity derivatives
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Asset	31,363	36,252
Liability	68,668	89,283
Fair Value, Measurements, Recurring [Member] \| Level 2 [Member] \| Foreign Exchange Contract [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Asset	15
Liability	15	45
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Assets, Fair Value Disclosure	123	31
Financial Liabilities Fair Value Disclosure	2,839	5,294
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Natural Gas Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Asset	123	31
Liability	2,227	2,805
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Power Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liability	612	2,488
Fair Value, Measurements, Recurring [Member] \| Level 3 [Member] \| Power Option Agreement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liability		1
Fixed Income Funds [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	1,758	1,745
Fixed Income Funds [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	1,758	1,745
Equity Funds [Member] \| Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	6,456	6,157
Equity Funds [Member] \| Fair Value, Measurements, Recurring [Member] \| Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	$ 6,456	$ 6,157