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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

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