Derivative instruments (Balance Sheet at Fair Value) (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair value of derivative instruments
Derivative assets, gross	$ 17,687	$ 6,811
Derivative liabilities, gross	(500)	(22,896)
Derivative assets (liabilities), net	17,187	(16,085)
Derivative assets, netting adjustments	(500)	(5,864)
Derivative liabilities, netting adjustments	500	5,864
Current derivative assets, net	15,197	947
Current derivative liabilities, net	0	(11,957)
Current derivative assets (liabilities), net	15,197	(11,010)
Long-term derivative assets, net	1,990	0
Long-term derivative liabilities, net	0	(5,075)
Long-term derivative assets (liabilities), net	1,990	(5,075)
Natural gas derivative contracts
Fair value of derivative instruments
Derivative assets, gross	2,288	3,552
Derivative liabilities, gross	(500)	(1)
Derivative assets (liabilities), net	1,788	3,551
Crude oil derivative contracts
Fair value of derivative instruments
Derivative assets, gross	15,399	391
Derivative liabilities, gross	0	(22,196)
Derivative assets (liabilities), net	15,399	(21,805)
NGL derivative contracts
Fair value of derivative instruments
Derivative assets, gross	0	2,868
Derivative liabilities, gross	0	(699)
Derivative assets (liabilities), net	$ 0	$ 2,169

Derivative Asset (Liability), Net, Current

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Derivative Asset (Liability), Net, Noncurrent

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Fair value of 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, not subject to a master netting arrangement, deducted from derivative assets.

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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 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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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, before 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 elected not to be offset. Excludes assets not subject to a master netting arrangement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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