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Financing: Short-Term Debt: (Details) $ in Millions | 12 Months Ended
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Dec. 31, 2023 USD ($) CreditExtension | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($)
Short-term Debt [Line Items] |  |  |
Amount outstanding | $ 0.0 | $ 535.6 |
Commercial Paper, Maximum Borrowing Capacity | 750.0 |  |
Term Loan - borrowings | 0.0 | 0.0 | $ 800.0
Repayments of Other Short-term Debt | $ 0.0 | 0.0 | $ 800.0
Commercial Paper |  |  |
Short-term Debt [Line Items] |  |  |
Debt Instrument, Term | 397 days |  |
Revolving Credit Facility |  |  |
Short-term Debt [Line Items] |  |  |
Line of Credit Facility, Current Borrowing Capacity | $ 750.0 |  |
Number Of One-Year Extension Options | CreditExtension | 2 |  |
Debt Instrument, Term | 1 year |  |
Maximum amount outstanding (based on daily outstanding balances) | $ 1,000.0 |  |
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage | 0.175% |  |
Debt Issuance Cost, Gross, Noncurrent | $ 8.9 |  |
Revolving Credit Facility | Base Rate |  |  |
Short-term Debt [Line Items] |  |  |
Weighted average interest rates | 0.125% |  |
Revolving Credit Facility | Eurodollar |  |  |
Short-term Debt [Line Items] |  |  |
Weighted average interest rates | 1.125% |  |
Revolving Credit Facility | Letter of Credit |  |  |
Short-term Debt [Line Items] |  |  |
Weighted average interest rates | 1.125% |  |
Revolving Credit Facility | Commercial Paper |  |  |
Short-term Debt [Line Items] |  |  |
Amount outstanding | $ 0.0 | 535.6 |
Letters of credit | 3.7 | 24.6 |
Available capacity | 746.3 | $ 189.8 |
Weighted average interest rates |  | 4.88% |
Maximum amount outstanding (based on daily outstanding balances) | 548.7 | $ 572.3 |
Average amount outstanding (based on daily outstanding balances) | $ 81.7 | $ 390.7 |
Weighted average interest rates | 4.91% | 2.11% |

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- Definition Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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