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Regulatory Accounting: (Details) - USD ($)
$ in Thousands
6 Months Ended 9 Months Ended
Jul. 01, 2017
Jun. 30, 2017
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Assets     $ 300,594 $ 296,142 $ 284,801
Regulatory Liabilities     $ 205,231 206,756 203,421
Approximate increase in annual amortization expense due to change in amortization periods $ 2,700        
Deferred energy and gas costs          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Liability, Amortization Period     1 year    
Regulatory Liabilities     $ 3,780 10,368 15,033
Employee benefit plans          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Liability, Amortization Period     12 years    
Regulatory Liabilities     $ 66,620 68,654 65,575
Cost of removal          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Liability, Amortization Period     44 years    
Regulatory Liabilities     $ 125,360 118,410 114,616
Revenue subject to refund          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Liability, Amortization Period     1 year    
Regulatory Liabilities     $ 1,386 2,485 1,892
Other regulatory liabilities          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Liability, Amortization Period     25 years    
Regulatory Liabilities     $ 8,085 6,839 6,305
Deferred energy and gas costs          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     1 year    
Regulatory Assets     $ 20,559 17,491 16,525
Deferred gas cost adjustments          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     1 year    
Regulatory Assets     $ 12,833 15,329 12,172
Gas price derivatives          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     3 years    
Regulatory Assets     $ 11,297 8,843 14,405
AFUDC          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     45 years    
Regulatory Assets     $ 15,645 15,227 14,093
Employee benefit plans          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     12 years    
Regulatory Assets     $ 105,671 108,556 107,578
Environmental          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Assets     $ 1,051 1,108 1,126
Asset retirement obligations          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     44 years    
Regulatory Assets     $ 514 505 507
Loss on reacquired debt          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     30 years    
Regulatory Assets     $ 21,067 22,266 18,077
Renewable energy standard adjustment          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     5 years    
Regulatory Assets     $ 1,956 1,605 1,694
Flow through accounting          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     35 years    
Regulatory Assets     $ 41,900 37,498 33,136
Decommissioning costs          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     6 years    
Regulatory Assets     $ 13,989 16,859 17,271
Decommissioning costs | South Dakota Public Utilities Commission (SDPUC)          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period 6 years        
Regulatory Assets   $ 11,000      
Decommissioning costs | South Dakota Public Utilities Commission (SDPUC) | Scenario, Previously Reported          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period   10 years      
Gas supply contract termination          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     5 years    
Regulatory Assets     $ 21,402 26,666 28,164
Other regulatory assets          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period     30 years    
Regulatory Assets     $ 32,710 $ 24,189 $ 20,053
Other regulatory assets | South Dakota Public Utilities Commission (SDPUC)          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period 6 years        
Regulatory Assets   $ 14,000      
Other regulatory assets | South Dakota Public Utilities Commission (SDPUC) | Winter Storm Atlas          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period 6 years        
Regulatory Assets   $ 2,000      
Other regulatory assets | South Dakota Public Utilities Commission (SDPUC) | Winter Storm Atlas | Scenario, Previously Reported          
Schedule Of Regulatory Assets And Liabilities [Line Items]          
Regulatory Asset, Amortization Period   10 years