Fair Value (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2024	Dec. 31, 2023
Long-term Debt	$ 1,838.2	$ 1,837.6
Subsidiaries [Member]
Long-term Debt	1,012.6	1,012.5
Level 1 [Member]
Total Assets	7.6	7.8
Level 1 [Member] \| Subsidiaries [Member]
Total Assets	7.6	7.8
Level 2 [Member]
Total Assets	0.0	0.0
Level 2 [Member] \| Subsidiaries [Member]
Total Assets	0.0	0.0
Level 3 [Member]
Total Assets	0.0	0.0
Level 3 [Member] \| Subsidiaries [Member]
Total Assets	0.0	0.0
Fair Value [Member]
Total Assets	7.6	7.8
Fair Value [Member] \| Subsidiaries [Member]
Total Assets	7.6	7.8
Money Market Funds [Member] \| Level 1 [Member]
Investments, Fair Value Disclosure	0.2	0.6
Money Market Funds [Member] \| Level 1 [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	0.2	0.6
Money Market Funds [Member] \| Level 2 [Member]
Investments, Fair Value Disclosure	0.0	0.0
Money Market Funds [Member] \| Level 2 [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	0.0	0.0
Money Market Funds [Member] \| Level 3 [Member]
Investments, Fair Value Disclosure	0.0	0.0
Money Market Funds [Member] \| Level 3 [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	0.0	0.0
Money Market Funds [Member] \| Fair Value [Member]
Investments, Fair Value Disclosure	0.2	0.6
Money Market Funds [Member] \| Fair Value [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	0.2	0.6
Debt [Member] \| Level 1 [Member]
Debt	0.0	0.0
Debt [Member] \| Level 1 [Member] \| Subsidiaries [Member]
Debt	0.0	0.0
Debt [Member] \| Level 2 [Member]
Debt	1,689.4	1,706.2
Debt [Member] \| Level 2 [Member] \| Subsidiaries [Member]
Debt	902.2	909.9
Debt [Member] \| Level 3 [Member]
Debt	16.8	16.8
Debt [Member] \| Level 3 [Member] \| Subsidiaries [Member]
Debt	16.8	16.8
Debt [Member] \| Fair Value [Member]
Debt	1,706.2	1,723.0
Debt [Member] \| Fair Value [Member] \| Subsidiaries [Member]
Debt	919.0	926.7
Mutual Fund \| Level 1 [Member]
Investments, Fair Value Disclosure	7.4	7.2
Mutual Fund \| Level 1 [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	7.4	7.2
Mutual Fund \| Level 2 [Member]
Investments, Fair Value Disclosure	0.0	0.0
Mutual Fund \| Level 2 [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	0.0	0.0
Mutual Fund \| Level 3 [Member]
Investments, Fair Value Disclosure	0.0	0.0
Mutual Fund \| Level 3 [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	0.0	0.0
Mutual Fund \| Fair Value [Member]
Investments, Fair Value Disclosure	7.4	7.2
Mutual Fund \| Fair Value [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	$ 7.4	$ 7.2

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value portion of debt instrument payable, including, but not limited to, notes payable and loans payable.

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Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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

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