Fair Value (Fair Value of Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2025	Dec. 31, 2024
Long-term Debt	$ 1,840.7	$ 1,840.0
Subsidiaries [Member]
Long-term Debt	1,013.0	1,012.9
Level 1 [Member]
Total Assets	7.4	8.0
Level 1 [Member] \| Subsidiaries [Member]
Total Assets	7.4	8.0
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	7.4	8.0
Fair Value [Member] \| Subsidiaries [Member]
Total Assets	7.4	8.0
Money Market Funds [Member] \| Level 1 [Member]
Investments, Fair Value Disclosure	0.6	0.4
Money Market Funds [Member] \| Level 1 [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	0.6	0.4
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	0.6	0.4
Money Market Funds [Member] \| Fair Value [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	0.6	0.4
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	1,711.7	1,692.4
Debt [Member] \| Level 2 [Member] \| Subsidiaries [Member]
Debt	901.7	894.8
Debt [Member] \| Level 3 [Member]
Debt	16.6	16.6
Debt [Member] \| Level 3 [Member] \| Subsidiaries [Member]
Debt	16.6	16.6
Debt [Member] \| Fair Value [Member]
Debt	1,728.3	1,709.0
Debt [Member] \| Fair Value [Member] \| Subsidiaries [Member]
Debt	918.3	911.4
Mutual Fund \| Level 1 [Member]
Investments, Fair Value Disclosure	0.6	7.6
Mutual Fund \| Level 1 [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	6.8	7.6
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	6.8	7.6
Mutual Fund \| Fair Value [Member] \| Subsidiaries [Member]
Investments, Fair Value Disclosure	$ 6.8	$ 7.6