Derivative Instruments - Fair value (Details) $ in Thousands	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Fair value of derivative instruments
Current portion of fair value of derivative assets	$ 245,263	$ 460,685
Noncurrent portion of fair value of derivative assets	362,169	841,257
Current portion of fair value of derivative liabilities	532	28,476
Noncurrent portion of fair value of derivative liabilities		207
Net derivatives	606,900	1,273,259
Commodity derivative
Fair value of derivative instruments
Total asset derivatives	607,432	1,301,942
Total liability derivatives	532	7,289
Marketing derivative
Fair value of derivative instruments
Total liability derivatives		21,394
Derivatives not designated as hedges for accounting purposes
Fair value of derivative instruments
Total asset derivatives	607,432	1,301,942
Total liability derivatives	532	28,683
Derivatives not designated as hedges for accounting purposes \| Commodity derivative
Fair value of derivative instruments
Current portion of fair value of derivative assets	245,263	460,685
Noncurrent portion of fair value of derivative assets	362,169	841,257
Current portion of fair value of derivative liabilities	$ 532	7,082
Noncurrent portion of fair value of derivative liabilities		207
Derivatives not designated as hedges for accounting purposes \| Marketing derivative
Fair value of derivative instruments
Current portion of fair value of derivative liabilities		$ 21,394
Derivatives designated as hedges for accounting purposes
Fair value of derivative instruments
Number of derivative instruments held designated as hedges \| item	0	0

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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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, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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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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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, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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The number of derivative instruments of a particular group held by the entity.

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