Derivative instruments (Derivative Instruments Recorded on the Balance Sheet at Fair Value) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Mar. 21, 2017
Fair value of derivative instruments
Derivative assets, gross	$ 6,811	$ 29,622
Derivative liabilities, gross	(22,896)	(4,940)
Derivative assets (liabilities), net	(16,085)	24,682
Netting adjustments, assets	(5,864)	(3,398)
Netting adjustments, liabilities	5,864	3,398
Current derivative assets, net	947	24,025	$ 19,058
Current derivative liabilities, net	(11,957)	0
Current derivative assets (liabilities), net	(11,010)	24,025
Long-term derivative assets, net	0	2,199	$ 14,295
Long-term derivative liabilities, net	(5,075)	(1,542)
Long-term derivative assets (liabilities), net	(5,075)	657
Natural gas derivative contracts
Fair value of derivative instruments
Derivative assets, gross	3,552	833
Derivative liabilities, gross	(1)	(488)
Derivative assets (liabilities), net	3,551	345
NGL derivative contracts
Fair value of derivative instruments
Derivative assets, gross	2,868	4,581
Derivative liabilities, gross	(699)	0
Derivative assets (liabilities), net	2,169	4,581
Crude oil derivative contracts
Fair value of derivative instruments
Derivative assets, gross	391	24,208
Derivative liabilities, gross	(22,196)	(4,452)
Derivative assets (liabilities), net	$ (21,805)	$ 19,756

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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