Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended			6 Months Ended
	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Jun. 30, 2025 USD ($) mi² customer segment	Jun. 30, 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	$ 70.1			$ 70.1			$ 54.0
Restricted Cash	0.1			$ 0.1			0.1
Number of Operating Segments | segment				1
Excise taxes collected	11.1	$ 11.7		$ 24.3	$ 24.0
Customer receivables	101.7			101.7			79.8
Unbilled Revenue	28.6			28.6			24.1
Accounts Receivable, Allowance for Credit Loss	(10.1)			(10.1)			(6.1)
Accounts Receivable, Allowance for Credit Loss, Current	10.1	1.0		10.1	1.0		6.1	$ 0.9
Accounts Receivable, Credit Loss Expense (Reversal)				3.6	2.3
Accounts Receivable, Allowance for Credit Loss, Writeoff				(0.3)	(2.7)
Accounts Receivable, Allowance for Credit Loss, Recovery				0.7	0.5
Interest expense	(19.9)	(20.8)		(43.8)	(41.1)
Income Tax Expense (Benefit)	(3.0)	(1.3)		(4.0)	(2.9)
Net income	(2.3)	(10.2)		(5.0)	(12.8)
Nonoperating Income (Expense)	18.9	18.9		43.4	37.4
Accumulated other comprehensive income	14.7			14.7			(5.3)
Other Comprehensive Income (Loss), Net of Tax	0.0	(0.2)	$ (0.2)	0.0	(0.4)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax				0.2
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				(0.2)
Contract with Customer, Asset, before Allowance for Credit Loss	130.3			130.3			103.9
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	70.2	48.5		70.2	48.5		54.1	$ 41.1
Accounts Receivable, after Allowance for Credit Loss, Current	137.1			137.1			106.8
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(0.2)	(0.3)		(0.4)	(0.5)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax	0.1	0.1		0.1	(0.0)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	0.1	(0.0)		0.1	(0.0)
Allowance for Funds Used During Construction, Equity Portion	0.1	2.2		0.1	4.0
Allowance for Funds Used During Construction, Capitalized Interest	(1.5)	(2.4)		(2.9)	(4.5)
Unrealized Gain (Loss) on Investments	0.3	$ 0.0		0.2	0.3
Proceeds from Issuance of Redeemable Convertible Preferred Stock	2.7			540.6	$ 0.0
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	(0.2)
Property, Plant and Equipment [Line Items]
Property, Plant and Equipment, Net	2,702.8			2,702.8			2,571.4
Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment	(607.6)			(607.6)			(571.7)
AOCI Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	17.5
525110 Pension Funds
Significant Accounting Policies [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	9.6
Cash Flow Hedging
Significant Accounting Policies [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	7.9			7.9
AOCI Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Other Comprehensive Income (Loss), before Tax, Portion Attributable to Parent	2.7
Nonrelated Party
Significant Accounting Policies [Line Items]
Other	13.8			13.8			6.0
Related Party
Significant Accounting Policies [Line Items]
Other	3.1			3.1			3.0
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	17.9			17.9			10.4
Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	$ (3.2)			$ (3.2)			$ (15.7)
Subsidiaries [Member]
Significant Accounting Policies [Line Items]
Approximate number of retail customers | customer				543,000
Service area, square miles | mi²				6,000
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%