Debt (Narrative) (Details) $ in Millions		9 Months Ended
	Apr. 13, 2023 USD ($)	Sep. 30, 2023 USD ($)	Sep. 30, 2022 USD ($)	Dec. 31, 2022 USD ($)
Debt Instrument [Line Items]
Long-term Line of Credit				$ 35.0
Proceeds from Issuance of Long-term Debt		$ 100.0	$ 140.0
Less: current portion		(0.2)		(0.2)
Debt Instrument, Unamortized Discount (Premium), Net		(0.7)		(0.7)
Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term Line of Credit		120.0		120.0
Proceeds from Issuance of Long-term Debt		100.0	$ 140.0
Less: current portion		(0.2)		(0.2)
Deferred Finance Costs, Net, Including Acquisition Adjustments		(6.7)		(6.9)
Debt Instrument, Unamortized Discount (Premium), Net		(2.3)		(2.4)
3.95% Senior Notes due 2049 [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 425.0		425.0
Debt Instrument, Interest Rate, Effective Percentage		3.95%
Revolving Credit Facility [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum		0.67
3.25% First Mortgage Bonds due 2040 [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 140.0		140.0
Debt Instrument, Interest Rate, Effective Percentage		3.20%
U.S. Government note maturing in February 2061 - 4.20% [Member] | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross				17.0
Debt instrument interest percentage		4.20%
4.25% Tax-exempt First Mortgage Bonds due 2027 | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 100.0		100.0
Debt Instrument, Interest Rate, Effective Percentage		4.25%
4.00% Tax-exempt First Mortgage Bonds due 2027 | Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term Debt, Gross		$ 40.0		$ 40.0
Debt Instrument, Interest Rate, Effective Percentage		4.00%
First Mortgage Bonds due 2033 | Subsidiaries [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of Long-term Debt	$ 100.0
Debt Instrument, Interest Rate, Effective Percentage	5.19%