Derivative instruments (Balance Sheet at Fair Value) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair value of derivative instruments
Derivative assets, gross	$ 16,100	$ 29,622
Derivative liabilities, gross	(6,635)	(4,940)
Derivative assets (liabilities), net	9,465	24,682
Derivative assets, netting adjustments	(3,543)	(3,398)
Derivative liabilities, netting adjustments	3,543	3,398
Current derivative assets, net	11,446	24,025
Current derivative liabilities, net	(70)	0
Current derivative assets (liabilities), net	11,376	24,025
Long-term derivative assets, net	1,111	2,199
Long-term derivative liabilities, net	(3,022)	(1,542)
Long-term derivative assets (liabilities), net	(1,911)	657
Natural gas derivative contracts
Fair value of derivative instruments
Derivative assets, gross	5,055	833
Derivative liabilities, gross	(40)	(488)
Derivative assets (liabilities), net	5,015	345
Crude oil derivative contracts
Fair value of derivative instruments
Derivative assets, gross	5,174	24,208
Derivative liabilities, gross	(6,164)	(4,452)
Derivative assets (liabilities), net	(990)	19,756
NGL derivative contracts
Fair value of derivative instruments
Derivative assets, gross	5,871	4,581
Derivative liabilities, gross	(431)	0
Derivative assets (liabilities), net	$ 5,440	$ 4,581

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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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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