Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2025 USD ($) mi² segment customer	Mar. 31, 2024 USD ($)	Apr. 04, 2025 USD ($)	Dec. 31, 2024 USD ($)	Dec. 31, 2023 USD ($)
Significant Accounting Policies [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 81.2			$ 54.0
Restricted Cash	$ 0.1			0.1
Number of Operating Segments | segment	1
Excise taxes collected	$ 13.2	$ 12.3
Customer receivables	102.9			79.8
Unbilled Revenue	26.3			24.1
Accounts Receivable, Allowance for Credit Loss	(8.1)			(6.1)
Accounts Receivable, Allowance for Credit Loss, Current	8.1	0.9		6.1	$ 0.9
Accounts Receivable, Credit Loss Expense (Reversal)	1.8	0.8
Accounts Receivable, Allowance for Credit Loss, Writeoff	(0.1)	(1.0)
Accounts Receivable, Allowance for Credit Loss, Recovery	0.3	0.2
Interest expense	(23.9)	(20.3)
Income Tax Expense (Benefit)	(1.0)	(1.6)
Net income	(2.7)	(2.6)
Nonoperating Income (Expense)	24.5	18.5
Accumulated other comprehensive income	(5.3)			(5.3)
Other Comprehensive Income (Loss), Net of Tax	0.0	(0.2)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.2	0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.0
Contract with Customer, Asset, before Allowance for Credit Loss	129.2			103.9
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	81.3	24.3		54.1	41.1
Accounts Receivable, after Allowance for Credit Loss, Current	131.4			106.8
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(0.2)	(0.2)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax	(0.0)	0.1
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	(0.0)	(0.0)
Allowance for Funds Used During Construction, Equity Portion	0.0	1.8
Allowance for Funds Used During Construction, Capitalized Interest	(1.4)	(2.1)
Unrealized Gain (Loss) on Investments	(0.1)	0.3
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	2,622.8			2,571.4
Forecast [Member]
Significant Accounting Policies [Line Items]
Ohio Holdings Purchase price			$ 272.0
Ohio Investments Purchase price			272.0
CDPQ Investment total			$ 544.0
Ohio Holdings Investment Percent			15.00%
Ohio Investment Percent			17.65%
CDPQ Investment percent			30.00%
Nonrelated Party
Significant Accounting Policies [Line Items]
Other	7.3			6.0
Related Party
Significant Accounting Policies [Line Items]
Other	3.0			3.0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	10.2			10.4
Other Comprehensive Income (Loss), Net of Tax	(0.2)
Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	(15.5)			(15.7)
Other Comprehensive Income (Loss), Net of Tax	$ 0.2
Subsidiaries [Member]
Significant Accounting Policies [Line Items]
Number of reportable segments | segment	1
Cash and Cash Equivalents, at Carrying Value	$ 60.4			24.6
Restricted Cash	$ 0.1			0.1
Approximate number of retail customers | customer	543,000
Service area, square miles | mi²	6,000
Number of Operating Segments | segment	1
Excise taxes collected	$ 13.2	12.3
Customer receivables	101.7			79.0
Unbilled Revenue	26.3			24.1
Accounts Receivable, Allowance for Credit Loss	(8.1)			(6.1)
Accounts Receivable, Allowance for Credit Loss, Current	8.1	0.9		6.1	0.9
Accounts Receivable, Credit Loss Expense (Reversal)	1.8	0.8
Accounts Receivable, Allowance for Credit Loss, Writeoff	0.1	1.0
Interest expense	(14.7)	(11.2)
Income Tax Expense (Benefit)	(1.7)	(2.3)
Net income	(10.7)	(9.4)
Nonoperating Income (Expense)	16.0	10.0
Accumulated other comprehensive income	(25.9)			(26.4)
Other Comprehensive Income (Loss), Net of Tax	0.5	0.4
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.5	0.4
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.5
Contract with Customer, Asset, before Allowance for Credit Loss	128.0			103.1
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	60.5	7.7		24.7	$ 15.6
Accounts Receivable, after Allowance for Credit Loss, Current	131.2			106.6
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax	(0.1)	(0.1)
Allowance for Funds Used During Construction, Equity Portion	0.0	1.8
Allowance for Funds Used During Construction, Capitalized Interest	(1.4)	(2.1)
Unrealized Gain (Loss) on Investments	(0.1)	$ 0.3
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	2,597.3			2,546.2
Subsidiaries [Member] | Forecast [Member]
Significant Accounting Policies [Line Items]
Ohio Holdings Investment Percent			15.00%
Ohio Investment Percent			17.65%
CDPQ Investment percent			30.00%
Subsidiaries [Member] | Nonrelated Party
Significant Accounting Policies [Line Items]
Other	7.4			6.0
Subsidiaries [Member] | Related Party
Significant Accounting Policies [Line Items]
Other	$ 3.9			$ 3.6