Derivatives (Schedule of Fair Value of Derivative Assets and Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Embedded Derivative, Fair Value, Liability	$ (48,600)
Preferred Distribution Rate Reset Election \| Other long-term liabilities
Derivatives, Fair Value [Line Items]
Embedded Derivative, Fair Value, Liability	(48,572)	$ (45,209)
Not Designated As Hedging Instrument \| Commodity Derivatives \| Current Assets - Other
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	1,481	505
Derivative Asset, Fair Value, Gross Liability	(1,481)	(505)
Derivative Asset	0	0
Derivative Liability, Fair Value, Gross Liability	(2,879)	(1,203)
Derivative Liability, Fair Value, Gross Asset	2,660	1,203
Derivative Liability	(219)	0
Not Designated As Hedging Instrument \| Swap \| Current Assets - Other
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	170	0
Not Designated As Hedging Instrument \| Swap \| Current Liabilities - Accrued Liabilities
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	(28)	0
Designated as Hedging Instrument \| Commodity Derivatives \| Current Assets - Other
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	28	54
Derivative Asset, Fair Value, Gross Liability	(28)	(54)
Derivative Asset	0	0
Derivative Liability, Fair Value, Gross Liability	(230)	(863)
Derivative Liability, Fair Value, Gross Asset	0	338
Derivative Liability	$ (230)	$ (525)

Fair value of liability associated with 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.

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Fair value, after the 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 not subject to a master netting arrangement and not elected to be offset.

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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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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 as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as a liability.

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