Income taxes (Rate Reconciliation) (Details)	3 Months Ended	9 Months Ended	12 Months Ended
Mar. 21, 2017	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Federal statutory rate	35.00%	35.00%	21.00%	21.00%
Remeasurement of deferred taxes—U.S. tax reform legislation	0.00%	(94.70%)	0.00%	0.00%
State remeasurement of deferred taxes	(0.00%)	(0.00%)	0.30%	(0.00%)
State income taxes, net of federal benefit	2.20%	5.80%	4.50%	(0.10%)
Statutory depletion	0.00%	0.40%	0.00%	(0.40%)
Valuation allowance	(25.90%)	54.10%	(25.00%)	2.80%
EOR tax credit	(0.00%)	(8.40%)	(0.00%)	(25.90%)
Return to provision adjustment	0.00%	10.20%	(0.60%)	(1.70%)
Other, net	(11.30%)	(2.40%)	(0.20%)	4.10%
Effective tax rate	0.00%	0.00%	0.00%	(0.20%)

Effective Income Tax Rate Reconciliation, Change In Deferred Tax Liabilities Valuation Allowance, Percent

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Effective Income Tax Rate Reconciliation, Enhanced Oil Recovery Tax Credit, Percent

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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