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Financing: Short-Term Debt: (Details) |  | 3 Months Ended | 12 Months Ended |
--- | --- | --- | --- | ---
Feb. 24, 2021 USD ($) | Mar. 31, 2021 USD ($) | Dec. 31, 2022 USD ($) CreditExtension | Dec. 31, 2021 USD ($) | Dec. 31, 2020 USD ($) | Jul. 19, 2021 USD ($)
Short-term Debt [Line Items] |  |  |  |  |  |
Amount outstanding |  |  | $ 535,600,000 | $ 420,180,000 |  |
Commercial Paper, Maximum Borrowing Capacity |  |  | 750,000,000 |  |  |
Term Loan - borrowings |  |  | 0 | 800,000,000 | $ 0 |
Repayments of Other Short-term Debt |  |  | $ 0 | 800,000,000 | $ 0 |
Commercial Paper |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |
Debt Instrument, Term |  |  | 397 days |  |  |
Line of Credit | Senior Unsecured Notes Due 2021 |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |
Debt Instrument, Term | 9 months |  |  |  |  |
Term Loan - borrowings | $ 800,000,000 |  |  |  |  |
Prepayment penalty |  |  | $ 0 |  |  |
Repayments of Other Short-term Debt |  | $ 200,000,000 |  |  |  |
Revolving Credit Facility |  |  |  |  |  |
Short-term Debt [Line Items] |  |  |  |  |  |
Line of Credit Facility, Current Borrowing Capacity |  |  |  |  |  | $ 750,000,000
Number Of One-Year Extension Options | CreditExtension |  |  | 2 |  |  |
Debt Instrument, Term |  |  | 1 year |  |  |
Maximum amount outstanding (based on daily outstanding balances) |  |  |  |  |  | $ 1,000,000,000.0
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage |  |  | 0.175% |  |  |
Debt Issuance Cost, Gross, Noncurrent |  |  | $ 8,900,000 |  |  |
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 |  |  | $ 535,600,000 | 420,180,000 |  |
Letters of credit |  |  | 24,626,000 | 27,209,000 |  |
Available capacity |  |  | 189,774,000 | 302,611,000 |  |
Maximum amount outstanding (based on daily outstanding balances) |  |  | 572,300,000 | 440,000,000 |  |
Average amount outstanding (based on daily outstanding balances) |  |  | $ 390,653,000 | $ 258,392,000 |  |
Weighted average interest rates |  |  | 2.11% | 0.22% |  |
Weighted average interest rates |  |  | 4.88% | 0.30% |  |

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