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Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions | Dec. 31, 2023 | Dec. 31, 2022
--- | --- | ---
Debt Instrument [Line Items] |  |
Long-term debt | $ 1,799.4 | $ 1,929.1
Unamortized Debt Issuance Costs | (8.1) | (8.3)
Total Long-Term Debt | 1,791.3 | 1,920.8
Less: Due Within One Year | 111.4 | 272.6
Net Long-Term Debt | $ 1,679.9 | 1,648.2
6.02% Series Due 2023 |  |
Debt Instrument [Line Items] |  |
Interest rate | 6.02% |
Long-term debt | $ 0.0 | 75.0
3.69% Series Due 2024 |  |
Debt Instrument [Line Items] |  |
Interest rate | 3.69% |
Long-term debt | $ 60.0 | 60.0
4.90% Series Due 2025 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.90% |
Long-term debt | $ 30.0 | 30.0
5.10% Series Due 2025 |  |
Debt Instrument [Line Items] |  |
Interest rate | 5.10% |
Long-term debt | $ 30.0 | 30.0
3.20% Series Due 2026 |  |
Debt Instrument [Line Items] |  |
Interest rate | 3.20% |
Long-term debt | $ 75.0 | 75.0
5.99% Series Due 2027 |  |
Debt Instrument [Line Items] |  |
Interest rate | 5.99% |
Long-term debt | $ 60.0 | 60.0
3.30% Series Due 2028 |  |
Debt Instrument [Line Items] |  |
Interest rate | 3.30% |
Long-term debt | $ 40.0 | 40.0
4.08% Series Due 2029 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.08% |
Long-term debt | $ 70.0 | 70.0
3.74% Series Due 2029 |  |
Debt Instrument [Line Items] |  |
Interest rate | 3.74% |
Long-term debt | $ 50.0 | 50.0
2.50% Series Due 2030 |  |
Debt Instrument [Line Items] |  |
Interest rate | 2.50% |
Long-term debt | $ 46.0 | 46.0
3.86% Series Due 2030 |  |
Debt Instrument [Line Items] |  |
Interest rate | 3.86% |
Long-term debt | $ 60.0 | 60.0
2.79% Series Due 2031 |  |
Debt Instrument [Line Items] |  |
Interest rate | 2.79% |
Long-term debt | $ 100.0 | 100.0
4.54% Series Due 2032 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.54% |
Long-term debt | $ 75.0 | 75.0
4.98% Series Due 2033 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.98% |
Long-term debt | $ 125.0 | 0.0
5.69% Series Due 2036 |  |
Debt Instrument [Line Items] |  |
Interest rate | 5.69% |
Long-term debt | $ 50.0 | 50.0
6.00% Series Due 2040 |  |
Debt Instrument [Line Items] |  |
Interest rate | 6.00% |
Long-term debt | $ 35.0 | 35.0
5.82% Series Due 2040 |  |
Debt Instrument [Line Items] |  |
Interest rate | 5.82% |
Long-term debt | $ 45.0 | 45.0
4.08% Series Due 2042 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.08% |
Long-term debt | $ 85.0 | 85.0
4.21% Series Due 2043 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.21% |
Long-term debt | $ 60.0 | 60.0
4.95% Series Due 2044 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.95% |
Long-term debt | $ 40.0 | 40.0
5.05% Series Due 2044 |  |
Debt Instrument [Line Items] |  |
Interest rate | 5.05% |
Long-term debt | $ 40.0 | 40.0
4.39% Series Due 2044 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.39% |
Long-term debt | $ 50.0 | 50.0
4.07% Series Due 2048 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.07% |
Long-term debt | $ 60.0 | 60.0
4.47% Series Due 2049 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.47% |
Long-term debt | $ 30.0 | 30.0
3.30% Series Due 2050 |  |
Debt Instrument [Line Items] |  |
Interest rate | 3.30% |
Long-term debt | $ 94.0 | 94.0
Armenia Mountain Senior Secured Notes 3.26% Due 2024 |  |
Debt Instrument [Line Items] |  |
Interest rate | 3.26% |
Long-term debt | $ 9.5 | 19.3
Industrial Development Variable Rate Demand Refunding Revenue Bonds Series 2006, Due 2025 |  |
Debt Instrument [Line Items] |  |
Long-term debt | 27.8 | 27.8
Revolving Credit Facility Variable Rate Due 2027 |  |
Debt Instrument [Line Items] |  |
Long-term debt | $ 0.0 | 13.0
Senior Unsecured Notes 2.65% Due 2025 |  |
Debt Instrument [Line Items] |  |
Interest rate | 2.65% |
Long-term debt | $ 150.0 | 150.0
Senior Unsecured Notes 3.11% Due 2027 |  |
Debt Instrument [Line Items] |  |
Interest rate | 3.11% |
Long-term debt | $ 80.0 | 80.0
SWL&P First Mortgage Bonds 4.15% Series Due 2028 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.15% |
Long-term debt | $ 15.0 | 15.0
SWL&P First Mortgage Bonds 4.14% Series Due 2048 |  |
Debt Instrument [Line Items] |  |
Interest rate | 4.14% |
Long-term debt | $ 12.0 | 12.0
Unsecured Term Loan Variable Rate Due 2023 |  |
Debt Instrument [Line Items] |  |
Long-term debt | $ 0.0 | 170.0
Other Long-Term Debt, 2023 Weighted Average Rate 5.24% Due 2024 – 2051 |  |
Debt Instrument [Line Items] |  |
Interest rate | 5.24% |
Long-term debt | $ 95.1 | $ 82.0

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Data Type: | na
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