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Long-term debt outstanding (Details 3) - USD ($)
$ in Thousands
Dec. 31, 2020
Oct. 30, 2020
Jun. 15, 2020
Dec. 31, 2019
Long-term debt outstanding [Line Items]        
Long-term debt $ 2,213,130     $ 2,243,107
Long-term debt due within one year 1,555     16,540
Long-term debt $ 2,211,575     2,226,567
Senior Notes        
Long-term debt outstanding [Line Items]        
Weighted average interest rate 4.40%      
Long-term debt $ 1,950,000     1,850,000
Senior Notes | Cascade Natural Gas [Member]        
Long-term debt outstanding [Line Items]        
Weighted average interest rate   3.34% 3.66%  
Long-term debt   $ 25,000 $ 50,000  
Commercial paper        
Long-term debt outstanding [Line Items]        
Weighted average interest rate 0.28%      
Long-term debt $ 125,600     222,900
Term Loan Agreements        
Long-term debt outstanding [Line Items]        
Weighted average interest rate 2.00%      
Long-term debt $ 8,400     9,100
Credit Agreements        
Long-term debt outstanding [Line Items]        
Weighted average interest rate 1.88%      
Long-term debt $ 95,900     89,050
Medium-term notes        
Long-term debt outstanding [Line Items]        
Weighted average interest rate 7.32%      
Long-term debt $ 35,000     50,000
Other notes        
Long-term debt outstanding [Line Items]        
Weighted average interest rate 0.75%      
Long-term debt $ 4,034     29,117
Long-term debt        
Long-term debt outstanding [Line Items]        
Unamortized debt issuance costs 5,803     7,010
Discount $ 1     $ 50