Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2024 USD ($)	Jun. 30, 2024 USD ($)	Mar. 31, 2024 USD ($)	Sep. 30, 2023 USD ($)	Jun. 30, 2023 USD ($)	Mar. 31, 2023 USD ($)	Sep. 30, 2024 USD ($) mi² customer segment	Sep. 30, 2023 USD ($)	Jan. 01, 2025 USD ($)	Dec. 31, 2023 USD ($)	Dec. 31, 2022 USD ($)
Significant Accounting Policies [Line Items]
Cash and Cash Equivalents, at Carrying Value	$ 41.0						$ 41.0			$ 41.0
Restricted Cash	0.1						0.1			0.1
Excise taxes collected	13.2			$ 12.8			37.2	$ 35.4
Customer receivables	75.5						75.5			71.0
Unbilled Revenue	18.8						18.8			19.4
Accounts Receivable, Allowance for Credit Loss	(1.8)						(1.8)			(0.9)
Accounts Receivable, Allowance for Credit Loss, Current	1.8			1.0			1.8	1.0		0.9	$ 0.5
Accounts Receivable, Credit Loss Expense (Reversal)							3.8	4.1
Accounts Receivable, Allowance for Credit Loss, Writeoff							(3.5)	(4.4)
Accounts Receivable, Allowance for Credit Loss, Recovery							0.6	0.8
Interest expense	(21.1)			(7.8)			(62.2)	(45.2)
Income Tax Expense (Benefit)	7.5			(9.6)			4.6	(5.3)
Net income	(10.3)	$ (10.2)	$ (2.6)	(38.6)	$ 5.2	$ (1.2)	(23.1)	(34.6)
Nonoperating Income (Expense)	19.7			6.4			57.1	40.5
Accumulated other comprehensive income	(5.1)						(5.1)			(4.6)
Other Comprehensive Income (Loss), Net of Tax	(0.1)	(0.2)	(0.2)	(0.2)	(0.1)	(0.2)	(0.5)	(0.5)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.0			0.0			0.1	0.1
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax							(0.5)
Contract with Customer, Asset, before Allowance for Credit Loss	94.3						94.3			90.4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	41.1			49.4			41.1	49.4		41.1	30.6
Accounts Receivable, after Allowance for Credit Loss, Current	101.4						101.4			92.5
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(0.1)			(0.2)			(0.6)	(0.6)
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax	(0.0)			(0.0)			(0.1)	(0.0)
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, Tax	0.1			(0.0)			0.1	0.1
Allowance for Funds Used During Construction, Equity Portion	1.4			1.3			5.4	3.8
Allowance for Funds Used During Construction, Capitalized Interest	(1.8)			(1.6)			(6.3)	(4.5)
Unrealized Gain (Loss) on Investments	0.4			(0.2)			0.7	0.2
Forecast [Member]
Significant Accounting Policies [Line Items]
Ohio Holdings Purchase price									$ 273.0
Ohio Investments Purchase price									273.0
CDPQ Investment total									$ 546.0
Ohio Holdings Investment Percent									15.00%
Ohio Investment Percent									17.65%
CDPQ Investment percent									30.00%
Nonrelated Party
Significant Accounting Policies [Line Items]
Other	6.7						6.7			2.2
Related Party
Significant Accounting Policies [Line Items]
Other	2.2						2.2			0.8
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	10.6						10.6			11.2
Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	(15.7)						$ (15.7)			(15.8)
Subsidiaries [Member]
Significant Accounting Policies [Line Items]
Number of reportable segments | segment							1
Cash and Cash Equivalents, at Carrying Value	19.6						$ 19.6			15.5
Restricted Cash	0.1						$ 0.1			0.1
Approximate number of retail customers | customer							541,000
Service area, square miles | mi²							6,000
Number of Operating Segments | segment							1
Excise taxes collected	13.2			12.8			$ 37.2	35.4
Customer receivables	74.6						74.6			70.0
Unbilled Revenue	18.8						18.8			19.4
Accounts Receivable, Allowance for Credit Loss	(1.8)						(1.8)			(0.9)
Accounts Receivable, Allowance for Credit Loss, Current	1.8			1.0			1.8	1.0		0.9	0.5
Accounts Receivable, Credit Loss Expense (Reversal)							3.8	4.1
Accounts Receivable, Allowance for Credit Loss, Writeoff							3.5	4.4
Interest expense	(11.8)			1.3			(34.7)	(16.7)
Income Tax Expense (Benefit)	1.9			(11.5)			(4.2)	(11.3)
Net income	(12.5)			(43.6)	0.4	(9.6)	(37.0)	(52.8)
Nonoperating Income (Expense)	11.1			(2.5)			31.5	12.7
Accumulated other comprehensive income	(26.7)						(26.7)			(27.9)
Other Comprehensive Income (Loss), Net of Tax	0.4	$ 0.4	$ 0.4	0.1	$ 0.1	$ 0.1	1.2	0.3
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.4			0.1			1.2	0.3
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax							1.2
Contract with Customer, Asset, before Allowance for Credit Loss	93.4						93.4			89.4
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	19.7			33.1			19.7	33.1		15.6	$ 19.8
Accounts Receivable, after Allowance for Credit Loss, Current	101.1						101.1			93.1
Other Comprehensive (Income) Loss, Defined Benefit Plan, Reclassification Adjustment from AOCI, Tax				(0.1)			(0.4)	(0.2)
Allowance for Funds Used During Construction, Equity Portion	1.4			1.3			5.4	3.8
Allowance for Funds Used During Construction, Capitalized Interest	(1.8)			(1.6)			(6.3)	(4.5)
Unrealized Gain (Loss) on Investments	0.4			$ (0.2)			0.7	$ 0.2
Subsidiaries [Member] | Forecast [Member]
Significant Accounting Policies [Line Items]
Ohio Holdings Purchase price									$ 273.0
Ohio Investments Purchase price									273.0
CDPQ Investment total									$ 546.0
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	6.6						6.6			2.2
Subsidiaries [Member] | Related Party
Significant Accounting Policies [Line Items]
Other	$ 2.9						$ 2.9			$ 2.4