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Oil and natural gas activities (unaudited) (Oil and Natural Gas Properties not Subject to Amortization) (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Mar. 21, 2017
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]      
Leasehold acreage $ 334,083    
Capitalized interest 16,785    
Wells in progress of completion 20,361    
Total unevaluated oil and natural gas properties excluded from amortization 371,229 $ 466,616 $ 585,574
Increase in fair value 235,723    
2019      
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]      
Leasehold acreage 5,223    
Capitalized interest 7,091    
Wells in progress of completion 20,361    
Total unevaluated oil and natural gas properties excluded from amortization 32,675    
2018      
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]      
Leasehold acreage 70,017    
Capitalized interest 9,694    
Wells in progress of completion 0    
Total unevaluated oil and natural gas properties excluded from amortization 79,711    
2017      
Costs Incurred, Oil and Gas Property Acquisition, Exploration, and Development Activities [Line Items]      
Leasehold acreage 258,843    
Capitalized interest 0    
Wells in progress of completion 0    
Total unevaluated oil and natural gas properties excluded from amortization $ 258,843