Property and equipment - Company property and equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Property, Plant and Equipment [Line Items]
Evaluated oil and natural gas properties	$ 7,773,776	$ 7,421,799
Less accumulated depletion and impairment	(6,669,537)	(5,725,114)
Evaluated oil and natural gas properties, net	1,104,239	1,696,685
Unevaluated oil and natural gas properties not being depleted	83,558	142,354
Midstream service assets, net	115,385	128,678
Depreciable other fixed assets, net	31,966	32,504
Property and equipment, net	1,335,148	2,000,221
Midstream service assets
Property, Plant and Equipment [Line Items]
Midstream service assets	182,169	180,932
Less accumulated depreciation and impairment	(66,784)	(52,254)
Depreciable other fixed assets
Property, Plant and Equipment [Line Items]
Less accumulated depreciation and impairment	(23,721)	(23,649)
Depreciable other fixed assets	36,786	37,894
Depreciable other fixed assets, net	13,065	14,245
Land
Property, Plant and Equipment [Line Items]
Depreciable other fixed assets, net	$ 18,901	$ 18,259

Capitalized costs or proved properties, net of accumulated depletion and impairment.

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Midstream Service Assets, Net

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Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

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Amount of accumulated depreciation, depletion, amortization and valuation allowance relating to oil and gas producing activities.

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The sum of the capitalized costs incurred, as of the balance sheet date, of unproved properties excluded from amortization including acquisition costs, exploration costs, development costs, and production costs.

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Capitalized costs of proved properties incurred for any combination mineral interests acquisitions; wells and related equipment; support equipment and facilities; and uncompleted wells and equipment and other costs not previously disclosed within this table.

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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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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Amount before accumulated depreciation, depletion and amortization of other physical assets used in the normal conduct of business to produce goods and services and not intended for resale.

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Amount after depreciation of long-lived, physical assets used to produce goods and services and not intended for resale, classified as other.

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