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Commitments: (Details) - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Operating leases      
2016 $ 5,318,000    
2017 5,318,000    
2018 5,318,000    
2019 2,950,000    
2020 588,000    
Rental expenses 4,849,000 $ 5,139,000 $ 5,213,000
Nuclear Fuel      
Fuel:      
2016 55,400,000    
2017 27,800,000    
2018 29,200,000    
2019 24,300,000    
2020 21,700,000    
Thereafter 60,800,000    
Public Utilities Inventory Coal      
Fuel:      
2016 29,658,000    
2017 15,342,000    
2018 $ 8,117,000