Derivative Instruments - Fair value (Details) $ in Thousands	Sep. 30, 2019 USD ($) item	Dec. 31, 2018 USD ($) item
Fair value of derivative instruments
Current portion of fair value of derivative assets	$ 411,774	$ 245,263
Noncurrent portion of fair value of derivative assets	405,180	362,169
Current portion of fair value of derivative liabilities		532
Commodity derivative
Fair value of derivative instruments
Total asset derivatives	816,954	607,432
Total liability derivatives		532
Derivatives not designated as hedges for accounting purposes
Fair value of derivative instruments
Total asset derivatives	816,954	607,432
Total liability derivatives		532
Net derivatives	816,954	606,900
Derivatives not designated as hedges for accounting purposes \| Commodity derivative
Fair value of derivative instruments
Current portion of fair value of derivative assets	411,774	245,263
Noncurrent portion of fair value of derivative assets	$ 405,180	362,169
Current portion of fair value of derivative liabilities		$ 532
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 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 within one year or 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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