Derivative Instruments and Hedging Activities (Summary Of Balance Sheet Locations And Related Fair Values Of Outstanding Derivative Instruments) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	$ 3,840	$ 9,484
Derivative Liability, Fair Value, Net Liability	6,804	32,580
Prepayments and other
Derivatives, Fair Value [Line Items]
Derivative, Fair Value, Net	3,840	9,484
Accrued Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Net Liability	5,365	32,580
Other Long Term Liabilities [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Net Liability	1,439
Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3,067	91
Derivative Asset, Fair Value, Gross Liability	0	0
Derivative, Fair Value, Net	3,067	91
Derivative Liability, Fair Value, Gross Liability	2,842	16,163
Derivative Liability, Fair Value, Gross Asset	0	431
Derivative Liability, Fair Value, Net Liability	2,842	15,732
Designated as Hedging Instrument [Member] \| Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability	2,424	13,185
Derivative Liability, Fair Value, Gross Asset	0	431
Derivative Liability, Fair Value, Net Liability	2,424	12,754
Designated as Hedging Instrument [Member] \| Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	3,067
Derivative Asset, Fair Value, Gross Liability	0
Derivative, Fair Value, Net	3,067
Derivative Liability, Fair Value, Gross Liability	418	2,978
Derivative Liability, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Net Liability	418	2,978
Designated as Hedging Instrument [Member] \| Interest rate swap
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		91
Derivative Asset, Fair Value, Gross Liability		0
Derivative, Fair Value, Net		91
Derivative Liability, Fair Value, Gross Liability		0
Derivative Liability, Fair Value, Gross Asset		0
Derivative Liability, Fair Value, Net Liability		0
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	773	10,149
Derivative Asset, Fair Value, Gross Liability	0	(756)
Derivative, Fair Value, Net	773	9,393
Derivative Liability, Fair Value, Gross Liability	3,962	26,735
Derivative Liability, Fair Value, Gross Asset	0	(9,887)
Derivative Liability, Fair Value, Net Liability	3,962	16,848
Not Designated as Hedging Instrument [Member] \| Commodity Contract [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		4,244
Derivative Asset, Fair Value, Gross Liability		(756)
Derivative, Fair Value, Net		3,488
Derivative Liability, Fair Value, Gross Liability		12,903
Derivative Liability, Fair Value, Gross Asset		(9,887)
Derivative Liability, Fair Value, Net Liability		3,016
Not Designated as Hedging Instrument [Member] \| Forward Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset	773	5,905
Derivative Asset, Fair Value, Gross Liability	0	0
Derivative, Fair Value, Net	773	5,905
Derivative Liability, Fair Value, Gross Liability	602	5,338
Derivative Liability, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Net Liability	602	5,338
Not Designated as Hedging Instrument [Member] \| Nymex Futures Contracts [Member]
Derivatives, Fair Value [Line Items]
Derivative Asset, Fair Value, Gross Asset		0
Derivative Asset, Fair Value, Gross Liability		0
Derivative, Fair Value, Net		0
Derivative Liability, Fair Value, Gross Liability	3,360	1,975
Derivative Liability, Fair Value, Gross Asset	0	0
Derivative Liability, Fair Value, Net Liability	$ 3,360	1,975
Not Designated as Hedging Instrument [Member] \| Foreign Exchange Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative Liability, Fair Value, Gross Liability		6,519
Derivative Liability, Fair Value, Gross Asset		0
Derivative Liability, Fair Value, Net Liability		$ 6,519

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, 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 of the assets less the liabilities of a derivative or group of derivatives.

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Fair value of asset associated with 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.

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The aggregate fair value amounts of derivative instruments that contain credit-risk-related contingent features that are in a net liability position at the end of the reporting period. For nonderivative instruments that are designated and qualify as hedging instruments, the fair value amounts are the carrying value of the nonderivative hedging instrument, including the adjustment for the foreign currency transaction gain (loss) on that instrument.

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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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