Derivative Instruments and Hedging Activities (Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Asset Fair Value	$ 4,289	$ 406
Liability Fair Value	(62,607)	(29,028)
Net Derivative Fair Value	(58,318)	(28,622)
Current assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	4,289	406
Current liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(47,167)	(27,744)
Long-term assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	0
Long-term liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(15,440)	(1,284)
Natural gas \| Current assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	750	406
Natural gas \| Current liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(21)	0
Natural gas \| Current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	729	406
Natural gas \| Long-term assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	0	0
Natural gas \| Long-term liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(1,299)	0
Natural gas \| Non-current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	(1,299)	0
Oil \| Current assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	3,539	0
Oil \| Current liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(47,146)	(27,744)
Oil \| Current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	(43,607)	(27,744)
Oil \| Long-term assets: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Asset Fair Value	0	0
Oil \| Long-term liabilities: Fair value of derivatives
Derivatives, Fair Value [Line Items]
Liability Fair Value	(14,141)	(1,284)
Oil \| Non-current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	$ (14,141)	$ (1,284)

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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