Fair Value (Narrative) (Details) - USD ($) $ in Thousands	6 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	$ 2,647	$ 2,586
Asset Retirement Obligation, Revision of Estimate	(7,717)
Asset retirement obligations	126,308	129,451
Unamortized deferred financing costs	21,900	23,000
Unamortized debt discount	6,600	6,700
Financial Transmission Rights Fair Value Disclosure	2,657	3,099
Assets, Fair Value Disclosure	5,304	5,685
Derivative Liability		53
Financial and Nonfinancial Liabilities, Fair Value Disclosure	23,054	53
Money Market Funds [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	19	21
Mutual Fund [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	2,628	2,565
Fair Value, Inputs, Level 3 [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	0	0
Financial Transmission Rights Fair Value Disclosure	2,657	3,099
Assets, Fair Value Disclosure	2,657	3,099
Derivative Liability		53
Financial and Nonfinancial Liabilities, Fair Value Disclosure	0	53
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]
Investments, Fair Value Disclosure	2,628	2,565
Financial Transmission Rights Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	2,628	2,565
Derivative Liability		0
Financial and Nonfinancial Liabilities, Fair Value Disclosure	23,054	0
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	2,628	2,565
Fair Value, Inputs, Level 1 [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	19	21
Financial Transmission Rights Fair Value Disclosure	0	0
Assets, Fair Value Disclosure	19	21
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	19	21
Fair Value, Inputs, Level 1 [Member] \| Mutual Fund [Member]
Fair Value, Option, Quantitative Disclosures [Line Items]
Investments, Fair Value Disclosure	$ 0	$ 0

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- Definition

Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation.

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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.

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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.

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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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