Income taxes (Components of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets related to
Asset retirement obligations	$ 8,500	$ 10,013
Accrued expenses, allowance and other	2,254	2,264
Derivative instruments	4,095	0
Net operating loss carryforwards
Federal	258,388	242,070
State	69,100	66,575
Statutory depletion carryforwards	2,351	2,383
Enhanced oil recovery credit	18,758	18,758
Interest limitation	4,153	5,771
Deferred tax assets, gross	367,599	347,834
Less valuation allowance	(336,123)	(216,109)
Deferred tax asset	31,476	131,725
Deferred tax liabilities related to
Property and equipment	(31,355)	(125,224)
Derivative instruments	0	(6,353)
Inventories	(121)	(148)
Deferred tax liability	(31,476)	(131,725)
Net deferred tax liability	$ 0	$ 0