|**Asset Retirement Obligations (Tables)**             | 12 Months Ended                                                                                                                                                                                                                                                                                                                                                                                                                  
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|**Asset Retirement Obligation Disclosure [Abstract]** |                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Change in asset retirement obligations                | The following table shows the change in our asset retirement obligations for2019and2018(dollars in thousands):20192018Asset retirement obligations at the beginning of year$726,545$679,529Changes attributable to:Accretion expense39,72636,876Settlements(12,591)(9,726)Estimated cash flow revisions(96,462)2,002Newly incurred  or acquired obligations—17,864Asset retirement obligations at the end of year$657,218$726,545
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|                                                      |                                                                                                                                                                                                                                                                                                                                                                                                                                  
|                                                      | 2019                                                                                                                                                                                                                                                                                                                                                                                                                             
|Asset retirement obligations at the beginning of year | $                                                                                                                                                                                                                                                                                                                                                                                                                                
|Changes attributable to:                              |                                                                                                                                                                                                                                                                                                                                                                                                                                  
|Accretion expense                                     | 39,726                                                                                                                                                                                                                                                                                                                                                                                                                           
|Settlements                                           | (12,591                                                                                                                                                                                                                                                                                                                                                                                                                          
|Estimated cash flow revisions                         | (96,462                                                                                                                                                                                                                                                                                                                                                                                                                          
|Newly incurred  or acquired obligations               | —                                                                                                                                                                                                                                                                                                                                                                                                                                
|Asset retirement obligations at the end of year       | $                                                                                                                                                                                                                                                                                                                                                                                                                                

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