Derivative Instruments and Hedging Activities (Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Asset Fair Value	$ 406	$ 103
Liability Fair Value	(29,028)	(18,296)
Total net liabilities	(28,622)	(18,193)
Current Assets
Derivatives, Fair Value [Line Items]
Asset Fair Value	406	103
Current Liabilities
Derivatives, Fair Value [Line Items]
Liability Fair Value	(27,744)	(18,268)
Non-current Liabilities
Derivatives, Fair Value [Line Items]
Liability Fair Value	(1,284)	(28)
Natural gas \| Current Assets
Derivatives, Fair Value [Line Items]
Asset Fair Value	406	0
Natural gas \| Current Liabilities
Derivatives, Fair Value [Line Items]
Liability Fair Value	0	(593)
Natural gas \| Derivative Current
Derivatives, Fair Value [Line Items]
Total net liabilities	406	(593)
Oil \| Current Assets
Derivatives, Fair Value [Line Items]
Asset Fair Value	0	103
Oil \| Current Liabilities
Derivatives, Fair Value [Line Items]
Liability Fair Value	(27,744)	(17,675)
Oil \| Derivative Current
Derivatives, Fair Value [Line Items]
Total net liabilities	(27,744)	(17,572)
Oil \| Non-current Assets
Derivatives, Fair Value [Line Items]
Asset Fair Value	0	0
Oil \| Non-current Liabilities
Derivatives, Fair Value [Line Items]
Liability Fair Value	(1,284)	(28)
Oil \| Derivative Noncurrent
Derivatives, Fair Value [Line Items]
Total net liabilities	$ (1,284)	$ (28)

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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- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

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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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No definition available.

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