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Financing: Schedule of Short-term Debt and Narrative (Details) - USD ($) $ in Thousands | Mar. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Short-term Debt [Line Items] |  |
Notes payable | $ 815,870 | $ 234,040
Letters of Credit | 16,629 | 24,730
Revolving Credit Facility |  |
Short-term Debt [Line Items] |  |
Notes payable | 0 | 0
Letters of Credit | 16,629 | 24,730
Term Loan |  |
Short-term Debt [Line Items] |  |
Notes payable | 600,000 | 0
Letters of Credit | 0 | 0
Commercial Paper |  |
Short-term Debt [Line Items] |  |
Notes payable | 215,870 | 234,040
Letters of Credit | $ 0 | $ 0

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