Fair Value (Narrative) (Details) $ in Thousands	6 Months Ended
Jun. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Fair Value, Option, Quantitative Disclosures [Line Items]
Financial Transmission Rights Fair Value Disclosure	$ 2,658	$ 543
Forward Power Contracts Fair Value Disclosure	2,651
Investments, Fair Value Disclosure	3,531	3,225
Asset retirement obligations	194,926	195,236
Unamortized deferred financing costs	24,900	26,000
Unamortized debt discount	6,500	6,600
Financial Transmission Rights Fair Value Disclosure	2,658	543
Assets, Fair Value Disclosure	8,840	3,768
Derivative Liability		63,215
Financial and Nonfinancial Liabilities, Fair Value Disclosure	$ 46,015	63,215
Forward Power Contracts Combined Notional Amount Outstanding	398,960
Money Market Funds [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	$ 16	16
Mutual Fund [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	3,515	3,209
Fair Value, Inputs, Level 3 [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Financial Transmission Rights Fair Value Disclosure	2,658	543
Forward Power Contracts Fair Value Disclosure	0
Investments, Fair Value Disclosure	0	0
Financial Transmission Rights Fair Value Disclosure	2,658	543
Assets, Fair Value Disclosure	2,658	543
Derivative Liability		0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 3 [Member] \| Money Market Funds [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 3 [Member] \| Mutual Fund [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 2 [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Financial Transmission Rights Fair Value Disclosure	0	0
Forward Power Contracts Fair Value Disclosure	2,651
Investments, Fair Value Disclosure	3,515	3,209
Financial Transmission Rights Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	6,166	3,209
Derivative Liability		63,215
Financial and Nonfinancial Liabilities, Fair Value Disclosure	46,015	63,215
Fair Value, Inputs, Level 2 [Member] \| Money Market Funds [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 2 [Member] \| Mutual Fund [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	3,515	3,209
Fair Value, Inputs, Level 1 [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Financial Transmission Rights Fair Value Disclosure	0	0
Forward Power Contracts Fair Value Disclosure	0
Investments, Fair Value Disclosure	16	16
Financial Transmission Rights Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	16	16
Derivative Liability		0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0	0
Fair Value, Inputs, Level 1 [Member] \| Money Market Funds [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	16	16
Fair Value, Inputs, Level 1 [Member] \| Mutual Fund [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	$ 0	$ 0

Financial Transmission Rights Fair Value Disclosure

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- Definition

Forward Power Contracts Combined Notional Amount Outstanding

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Forward Power Contracts Fair Value Disclosure

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Noncurrent portion of the carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Amount, after accumulated amortization, of debt discount.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Fair value of financial and nonfinancial obligations.

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