Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2021 USD ($) mi² customer segment	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Significant Accounting Policies [Line Items]
Number of reportable segments | segment							1
Cash and Cash Equivalents, at Carrying Value	$ 41.5						$ 41.5		$ 25.4
Excise taxes collected	13.5			$ 13.4			37.7	$ 36.7
Restricted Cash and Cash Equivalents, Current	0.1						0.1		0.1
Restricted Cash and Cash Equivalents	41.6			43.6			41.6	43.6	25.5	$ 47.0
Customer receivables	47.3						47.3		48.5
Unbilled Revenue	15.6						15.6		21.6
Amounts due from partners in jointly owned stations	1.5						1.5		0.2
Due from PJM transmission settlement	1.7						1.7		1.7
Other	3.1						3.1		0.5
Accounts Receivable, Allowance for Credit Loss	(0.9)						(0.9)		(2.8)
Accounts receivable, net	68.3						68.3		69.7
Accounts Receivable, Allowance for Credit Loss, Current	0.9			3.2			0.9	3.2	2.8	0.4
Accounts Receivable, Credit Loss Expense (Reversal)							(0.4)	2.8
Accounts Receivable, Allowance for Credit Loss, Writeoff							(2.6)	(1.4)
Accounts Receivable, Allowance for Credit Loss, Recovery							1.1	1.4
Interest and Debt Expense	(15.8)			(17.1)			(47.0)	(55.4)
Income Tax Expense (Benefit)	0.8			10.4			0.2	7.7
Net income	8.5		$ 13.6	(8.2)		$ 1.9	22.5	(2.8)
Nonoperating Income (Expense)	(16.6)			(47.8)			(46.7)	(86.2)
Income tax expense / (benefit) from discontinued operations	0.0			0.3			(0.2)	0.8
Accumulated other comprehensive income	(11.4)						(11.4)		(12.3)
Other Comprehensive Income (Loss), Net of Tax	0.3	$ 0.3	$ 0.3	0.1		(0.3)	0.9	0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent							(0.9)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.2)			(0.3)			(0.6)	(0.8)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.5			0.3			1.5	0.8
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	0.0			0.0			0.1	0.1
Contract with Customer, Asset, before Allowance for Credit Loss	62.9						62.9		70.1
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	13.0						13.0		13.6
Other Comprehensive Income (Loss), Net of Tax							(0.6)
Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	(24.4)						(24.4)		(25.9)
Other Comprehensive Income (Loss), Net of Tax							1.5
Reclassification out of Accumulated Other Comprehensive Income [Member]
Significant Accounting Policies [Line Items]
Net income	0.3			0.0			0.9	0.0
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 and Debt Expense	(0.2)			(0.3)			(0.7)	(0.9)
Income Tax Expense (Benefit)				0.0				0.1
Net income				(0.3)				(0.8)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Income Tax Expense (Benefit)				0.0				(0.1)
Net income				0.3				0.8
Nonoperating Income (Expense)	0.6			0.3			$ 1.9	0.9
Subsidiaries [Member]
Significant Accounting Policies [Line Items]
Number of reportable segments | segment							1
Cash and Cash Equivalents, at Carrying Value	29.8						$ 29.8		11.7
Approximate number of retail customers | customer							533,000
Service area, square miles | mi²							6,000
Number of Operating Segments | segment							1
Excise taxes collected	13.5			13.4			$ 37.7	36.7
Restricted Cash and Cash Equivalents, Current	0.1						0.1		0.1
Restricted Cash and Cash Equivalents	29.9			26.2			29.9	26.2	11.8	21.3
Customer receivables	46.6						46.6		47.6
Unbilled Revenue	15.6						15.6		21.6
Amounts due from partners in jointly owned stations	2.3						2.3		1.9
Due from PJM transmission settlement	1.7						1.7		1.7
Other	3.2						3.2		0.2
Accounts Receivable, Allowance for Credit Loss	(0.9)						(0.9)		(2.8)
Accounts receivable, net	68.5						68.5		70.2
Accounts Receivable, Allowance for Credit Loss, Current	0.9			3.2			0.9	3.2	2.8	$ 0.4
Accounts Receivable, Credit Loss Expense (Reversal)							(0.4)	2.8
Accounts Receivable, Allowance for Credit Loss, Writeoff							2.6	1.4
Interest and Debt Expense	(6.1)			(5.9)			(18.1)	(18.5)
Income Tax Expense (Benefit)	(2.7)			(3.5)			(6.4)	(2.4)
Net income	13.4			17.8			40.3	47.3
Nonoperating Income (Expense)	(7.4)			(5.7)			(19.7)	(19.9)
Accumulated other comprehensive income	(39.2)						(39.2)		(42.1)
Other Comprehensive Income (Loss), Net of Tax	1.0			0.9	$ 0.9	$ 0.4	2.9	2.2
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent							(2.9)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	0.0			(0.1)			0.0	(0.3)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	1.0			0.8			2.9	2.4
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	0.0			(0.1)			0.0	(0.1)
Income tax (expense)/benefit on reclassification of earnings related to pension and postretirement activity	(0.3)			(0.2)			(0.9)	(0.6)
Contract with Customer, Asset, before Allowance for Credit Loss	62.2						62.2		$ 69.2
Subsidiaries [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Significant Accounting Policies [Line Items]
Net income	1.0			0.7			2.9	2.1
Subsidiaries [Member] | 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 and Debt Expense	0.0			0.0			0.0	(0.2)
Income Tax Expense (Benefit)				(0.1)				(0.1)
Net income	0.0			(0.1)			0.0	(0.3)
Subsidiaries [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Income Tax Expense (Benefit)				(0.2)				(0.6)
Net income	1.0			0.8			2.9	2.4
Nonoperating Income (Expense)	$ 1.3			$ 1.0			$ 3.8	$ 3.0