Derivative Instruments and Hedging Activities (Fair Value of Derivative Instruments) (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivatives, Fair Value [Line Items]
Asset Fair Value	$ 24,843	$ 65,114
Liability Fair Value	(20,914)	(17,920)
Net Derivative Fair Value	3,929	47,194
Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Asset Fair Value	10,679	65,114
Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Liability Fair Value	(17,251)	(10,480)
Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Asset Fair Value	2,339
Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Liability Fair Value	(3,663)	(7,440)
Commodity - Oil \| Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Asset Fair Value	8,671	60,097
Commodity - Oil \| Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Liability Fair Value	(17,251)	(10,480)
Commodity - Oil \| Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	(8,580)	49,617
Commodity - Oil \| Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Asset Fair Value	2,335	0
Commodity - Oil \| Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Liability Fair Value	(3,327)	(5,672)
Commodity - Oil \| Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	(992)	(5,672)
Commodity - Natural gas \| Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Asset Fair Value	2,008	5,017
Commodity - Natural gas \| Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Liability Fair Value	0	0
Commodity - Natural gas \| Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	2,008	5,017
Commodity - Natural gas \| Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Asset Fair Value	4	0
Commodity - Natural gas \| Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Liability Fair Value	(336)	(1,768)
Commodity - Natural gas \| Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	(332)	$ (1,768)
Contingent consideration arrangement \| Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Asset Fair Value	2,485
Contingent consideration arrangement \| Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Liability Fair Value	0
Contingent consideration arrangement \| Fair value of derivatives - Current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	2,485
Contingent consideration arrangement \| Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Asset Fair Value	9,340
Contingent consideration arrangement \| Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Liability Fair Value	0
Contingent consideration arrangement \| Fair value of derivatives - Non-current
Derivatives, Fair Value [Line Items]
Net Derivative Fair Value	$ 9,340

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 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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