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Subsequent events (Details)
$ in Millions
6 Months Ended 12 Months Ended
Aug. 06, 2019
USD ($)
Jul. 22, 2019
USD ($)
a
Jun. 30, 2019
USD ($)
Dec. 31, 2018
USD ($)
Aug. 05, 2019
USD ($)
Jul. 19, 2019
USD ($)
Subsequent events            
Repayment of credit facility     $ 210.0 $ 27.0    
Amount outstanding     $ 435.0 $ 210.0    
Subsequent repayment under credit facility            
Subsequent events            
Repayment of credit facility $ 50.0          
Amount outstanding $ 225.0       $ 275.0  
Marcellus Shale Royalties Pennsylvania            
Subsequent events            
Purchase price   $ 300.0        
Percentage of gross production less deduction   1.00%        
Net acres of production | a   350,000        
Royalty generated   $ 9.0        
Subsequent authorization for issuance of shares from treasury to public            
Subsequent events            
Maximum value of shares that can be issued from treasury to the public           $ 200.0