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Short-Term and Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Millions	Dec. 31, 2024	Dec. 31, 2023
Debt Instrument [Line Items]
Long-term debt	$ 1,808.0	$ 1,799.4
Unamortized Debt Issuance Costs	(8.6)	(8.1)
Total Long-Term Debt	1,799.4	1,791.3
Less: Due Within One Year	94.7	111.4
Net Long-Term Debt	$ 1,704.7	1,679.9
3.69% Series Due 2024
Debt Instrument [Line Items]
Interest rate	3.69%
Long-term debt	$ 0.0	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	125.0
5.69% Series Due 2036
Debt Instrument [Line Items]
Interest rate	5.69%
Long-term debt	$ 50.0	50.0
5.72% Series Due 2039
Debt Instrument [Line Items]
Interest rate	5.72%
Long-term debt	$ 100.0	0.0
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	$ 0.0	9.5
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	$ 20.0	0.0
Senior Unsecured Notes 2.65% Due 2025
Debt Instrument [Line Items]
Interest rate	2.65%
Long-term debt	$ 0.0	150.0
Senior Unsecured Notes 3.11% Due 2027
Debt Instrument [Line Items]
Interest rate	3.11%
Long-term debt	$ 80.0	80.0
Senior Unsecured Notes 5.94% Due 2029
Debt Instrument [Line Items]
Interest rate	5.94%
Long-term debt	$ 100.0	0.0
Senior Unsecured Notes 6.18% Due 2034
Debt Instrument [Line Items]
Interest rate	6.18%
Long-term debt	$ 50.0	0.0
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
Other Long-Term Debt, 2024 Weighted Average Rate 4.47% Due 2025 – 2051
Debt Instrument [Line Items]
Interest rate	4.47%
Long-term debt	$ 53.2	$ 95.1

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