Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2022 USD ($)	Jun. 30, 2022 USD ($)	Mar. 31, 2022 USD ($)	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2022 USD ($) mi² customer segment	Sep. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment							1
Cash and Cash Equivalents, at Carrying Value	$ 44.3						$ 44.3		$ 26.6
Restricted Cash	0.1						0.1		0.1
Excise taxes collected	13.0			$ 13.5			37.6	$ 37.7
Customer receivables	66.0						66.0		42.3
Unbilled Revenue	15.0						15.0		19.2
Amounts due from partners in jointly owned stations	4.6						4.6		3.1
Due from PJM transmission settlement	1.7						1.7		1.7
Other	2.7						2.7		5.5
Accounts Receivable, Allowance for Credit Loss	(0.4)						(0.4)		(0.3)
Accounts Receivable, Allowance for Credit Loss, Current	0.4			0.9			0.4	0.9	0.3	$ 2.8
Accounts Receivable, Credit Loss Expense (Reversal)							1.4	(0.4)
Accounts Receivable, Allowance for Credit Loss, Writeoff							(2.1)	(2.6)
Accounts Receivable, Allowance for Credit Loss, Recovery							0.8	1.1
Interest expense	(18.1)			(15.8)			(50.1)	(47.0)
Income Tax Expense (Benefit)	4.1			0.8			8.6	0.2
Net income	19.7	$ (0.2)	$ (15.4)	(8.5)	$ (0.4)	$ (13.6)	4.1	(22.5)
Nonoperating Income (Expense)	16.5			16.6			48.1	46.7
Income tax benefit from discontinued operations	0.0			0.0			0.0	(0.2)
Accumulated other comprehensive income	(4.6)						(4.6)		(4.8)
Other Comprehensive Income (Loss), Net of Tax	0.1	$ 0.0	$ 0.1	0.3	0.3	0.3	0.2	0.9
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.2			0.5			0.7	1.5
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.1			0.3			0.2	0.9
Contract with Customer, Asset, before Allowance for Credit Loss	81.0						81.0		61.5
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	44.4			41.6			44.4	41.6	26.7	25.5
Accounts Receivable, after Allowance for Credit Loss, Current	89.6						89.6		71.5
Other Comprehensive Income (Loss), Cash Flow Hedge, Gain (Loss), Reclassification, after Tax	(0.1)			(0.2)			(0.5)	(0.6)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	12.3						12.3		12.8
Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	(16.9)						(16.9)		(17.6)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Interest expense	(0.2)			(0.2)			(0.7)	(0.7)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Nonoperating Income (Expense)	0.3			0.6			$ 0.9	1.9
Subsidiaries [Member]
Significant Accounting Policies [Line Items]
Number of reportable segments | segment							1
Cash and Cash Equivalents, at Carrying Value	27.6						$ 27.6		14.4
Restricted Cash	0.1						$ 0.1		0.1
Approximate number of retail customers | customer							535,000
Service area, square miles | mi²							6,000
Number of Operating Segments | segment							1
Excise taxes collected	13.0			13.5			$ 37.6	37.7
Customer receivables	65.0						65.0		41.6
Unbilled Revenue	15.0						15.0		19.2
Amounts due from partners in jointly owned stations	5.7						5.7		4.4
Due from PJM transmission settlement	1.7						1.7		1.7
Other	2.6						2.6		5.3
Accounts Receivable, Allowance for Credit Loss	(0.4)						(0.4)		(0.3)
Accounts Receivable, Allowance for Credit Loss, Current	0.4			0.9			0.4	0.9	0.3	2.8
Accounts Receivable, Credit Loss Expense (Reversal)							1.4	(0.4)
Accounts Receivable, Allowance for Credit Loss, Writeoff							2.1	2.6
Interest expense	(8.4)			(6.1)			(21.1)	(18.1)
Income Tax Expense (Benefit)	5.5			(2.7)			2.8	(6.4)
Net income	10.1			(13.4)	(8.6)	(18.3)	(14.1)	(40.3)
Nonoperating Income (Expense)	7.3			7.4			20.9	19.7
Accumulated other comprehensive income	(29.8)						(29.8)		(31.8)
Other Comprehensive Income (Loss), Net of Tax	0.6			1.0	$ 0.9	$ 1.0	2.0	2.9
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent							2.0
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.6			1.0			2.0	2.9
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax	0.6			1.0			2.0	2.9
Contract with Customer, Asset, before Allowance for Credit Loss	80.0						80.0		60.8
Cash, Cash Equivalents, Restricted Cash and Restricted Cash Equivalents	27.7			29.9			27.7	29.9	14.5	$ 11.8
Accounts Receivable, after Allowance for Credit Loss, Current	89.6						89.6		$ 71.9
Subsidiaries [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Net income	0.6			1.0			2.0	2.9
Nonoperating Income (Expense)	$ 0.8			$ 1.3			$ 2.6	$ 3.8