Derivative Instruments - Fair value (Details) $ in Thousands	Sep. 30, 2018 USD ($) item	Dec. 31, 2017 USD ($) item
Fair value of derivative instruments
Current portion of fair value of derivative assets	$ 493,354	$ 460,685
Noncurrent portion of fair value of derivative assets	672,768	841,257
Current portion of fair value of derivative liabilities	10,456	28,476
Noncurrent portion of fair value of derivative liabilities		207
Commodity derivative
Fair value of derivative instruments
Total asset derivatives	1,166,122	1,301,942
Total liability derivatives	5,045	7,289
Marketing derivative
Fair value of derivative instruments
Total liability derivatives	5,411	21,394
Derivatives not designated as hedges for accounting purposes
Fair value of derivative instruments
Total asset derivatives	1,166,122	1,301,942
Total liability derivatives	10,456	28,683
Net derivatives	1,155,666	1,273,259
Derivatives not designated as hedges for accounting purposes \| Commodity derivative
Fair value of derivative instruments
Current portion of fair value of derivative assets	493,354	460,685
Noncurrent portion of fair value of derivative assets	672,768	841,257
Current portion of fair value of derivative liabilities	5,045	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	$ 5,411	$ 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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