Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

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The estimated net amount of existing gains or losses on cash flow hedges at the reporting date expected to be reclassified to earnings within the next 12 months.

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Fair value, before effects of master netting arrangements, of a financial asset or other 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 assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Fair value, before 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 elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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Fair value of the assets less the liabilities of a derivative or group of derivatives.

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Net gain (loss) relating to components of the gain (loss) on the fair value hedging instrument excluded from the assessment of fair value hedge effectiveness. Recognized in earnings.

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Amount of right to receive cash and securities on deposit with broker-dealers to demonstrate an intention to complete a purchase (good faith deposit) and collateral deposited by counterparty to cover the credit risk assumed (margin deposit).

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Derivative Financial Instruments (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Derivative [Line Items]
Energy commodity derivatives deposits		$ 35,610		$ 36,690
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months		(3,000)
Commodity Contract [Member]
Derivative [Line Items]
Gain (Loss) from Components Excluded from Assessment of Fair Value Hedge Effectiveness, Net	$ 300		$ 1,700
Cash Flow Hedging [Member] \| Interest Rate Contract [Member]
Derivative [Line Items]
Derivative, Fair Value, Net		19,300
Cash Flow Hedging [Member] \| Interest Rate Contract [Member] \| Other Current Assets [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		19,500
Cash Flow Hedging [Member] \| Interest Rate Contract [Member] \| Other Noncurrent Assets [Member]
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset		200
Cash Flow Hedging [Member] \| Interest Rate Contract [Member] \| Other Noncurrent Liabilities [Member]
Derivative [Line Items]
Derivative Liability, Fair Value, Gross Liability		400
Future Debt Issuance 2019 [Member] \| Cash Flow Hedging [Member] \| Interest Rate Contract [Member]
Derivative [Line Items]
Derivative, Notional Amount		100,000
Future Debt Issuance 2018 [Member] \| Cash Flow Hedging [Member] \| Interest Rate Contract [Member]
Derivative [Line Items]
Derivative, Notional Amount		$ 100,000