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Business Segment Information: Capital Expenditures and Asset Acquisitions (Details) - USD ($)
$ in Thousands
12 Months Ended
Feb. 12, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Segment Reporting Information [Line Items]        
Capital Expenditures and Asset Acquisitions   $ 337,689 $ 460,450  
Acquisition of net assets, net of long-term debt assumed $ 1,124,238 0 (1,124,238) $ (21,970)
Total Capital Expenditures And Asset Acquisitions Of Continuing Operations   337,689 1,584,688  
Capital Expenditure, Discontinued Operations   23,222 6,669  
Property, Plant and Equipment Including New Asset Acquisitions, Gross Period Increase (Decrease)   360,911 1,591,357  
Gas Utilities        
Segment Reporting Information [Line Items]        
Acquisition of net assets, net of long-term debt assumed   0 1,124,238  
Source Gas        
Segment Reporting Information [Line Items]        
Capital Expenditures and Asset Acquisitions $ 1,124,000      
Corporate, Non-Segment        
Segment Reporting Information [Line Items]        
Capital Expenditures and Asset Acquisitions   6,668 17,353  
Electric Utilities        
Segment Reporting Information [Line Items]        
Capital Expenditures and Asset Acquisitions   138,060 258,739  
Gas Utilities        
Segment Reporting Information [Line Items]        
Capital Expenditures and Asset Acquisitions   184,389 173,930  
Power Generation        
Segment Reporting Information [Line Items]        
Capital Expenditures and Asset Acquisitions   1,864 4,719  
Mining        
Segment Reporting Information [Line Items]        
Capital Expenditures and Asset Acquisitions   $ 6,708 $ 5,709