Debt (Narrative) (Details) $ in Millions	3 Months Ended
Mar. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Debt Instrument [Line Items]
Less: current portion	$ (0.2)	$ (0.2)
Debt Instrument, Unamortized Discount (Premium), Net	(0.6)	(0.6)
Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term Line of Credit	170.0	15.0
Less: current portion	(0.2)	(0.2)
Deferred Finance Costs, Net, Including Acquisition Adjustments	(6.9)	(7.1)
Debt Instrument, Unamortized Discount (Premium), Net	(2.3)	(2.2)
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		16.8
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%

Debt Covenant, Total Debt to Total Capitalization Ratio, Maximum

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- Definition

Deferred Finance Costs, Net, Including Acquisition Adjustments

+ References

No definition available.

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Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Amount, after accumulated amortization, of debt discount (premium).

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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Amount, after deduction of unamortized premium (discount) and debt issuance cost, of long-term debt classified as current. Excludes lease obligation.

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