Summary of Significant Accounting Policies (Narrative) (Details) $ in Millions	3 Months Ended
	Mar. 31, 2022 USD ($) mi² customer segment	Mar. 31, 2021 USD ($)	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	$ 32.1		$ 26.6
Restricted Cash	0.1		0.1
Excise taxes collected	13.1	$ 12.9
Restricted Cash and Cash Equivalents	32.2	34.2	26.7	$ 25.5
Customer receivables	47.4		42.3
Unbilled Revenue	17.0		19.2
Amounts due from partners in jointly owned stations	3.7		3.1
Due from PJM transmission settlement	1.7		1.7
Other	4.3		5.5
Accounts Receivable, Allowance for Credit Loss	(0.2)		(0.3)
Accounts receivable, net	73.9		71.5
Accounts Receivable, Allowance for Credit Loss, Current	0.2	2.3	0.3	2.8
Accounts Receivable, Credit Loss Expense (Reversal)	(0.1)	0.0
Accounts Receivable, Allowance for Credit Loss, Writeoff	(0.2)	(0.7)
Accounts Receivable, Allowance for Credit Loss, Recovery	0.2	0.2
Interest and Debt Expense	(15.5)	(15.6)
Income Tax Expense (Benefit)	3.7	0.3
Net income	15.4	13.6
Nonoperating Income (Expense)	(15.1)	(15.1)
Income tax benefit from discontinued operations	0.0	(0.2)
Accumulated other comprehensive income	(4.7)		(4.8)
Other Comprehensive Income (Loss), Net of Tax	0.1	0.3
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.1)
Other Comprehensive Income (Loss), Reclassification Adjustment from AOCI on Derivatives, Net of Tax	(0.1)	(0.2)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.2	0.5
Income tax (expense)/benefit on reclassification of earnings related to derivative activity	0.1	0.0
Contract with Customer, Asset, before Allowance for Credit Loss	64.4		61.5
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	12.7		12.8
Other Comprehensive Income (Loss), Net of Tax	(0.1)
Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Accumulated other comprehensive income	(17.4)		(17.6)
Other Comprehensive Income (Loss), Net of Tax	0.2
Reclassification out of Accumulated Other Comprehensive Income [Member]
Significant Accounting Policies [Line Items]
Net income	0.1	0.3
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)	(0.2)
Income Tax Expense (Benefit)		0.0
Net income		(0.2)
Reclassification out of Accumulated Other Comprehensive Income [Member] | Change in unfunded pension obligation [Member]
Significant Accounting Policies [Line Items]
Income Tax Expense (Benefit)		(0.1)
Net income		0.5
Nonoperating Income (Expense)	$ 0.3	0.6
Subsidiaries [Member]
Significant Accounting Policies [Line Items]
Number of reportable segments | segment	1
Cash and Cash Equivalents, at Carrying Value	$ 17.9		14.4
Restricted Cash	$ 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	$ 13.1	12.9
Restricted Cash and Cash Equivalents	18.0	16.9	14.5	11.8
Customer receivables	46.5		41.6
Unbilled Revenue	17.0		19.2
Amounts due from partners in jointly owned stations	4.3		4.4
Due from PJM transmission settlement	1.7		1.7
Other	4.3		5.3
Accounts Receivable, Allowance for Credit Loss	(0.2)		(0.3)
Accounts receivable, net	73.6		71.9
Accounts Receivable, Allowance for Credit Loss, Current	0.2	2.3	0.3	$ 2.8
Accounts Receivable, Credit Loss Expense (Reversal)	(0.1)	0.0
Accounts Receivable, Allowance for Credit Loss, Writeoff	0.2	0.7
Interest and Debt Expense	(5.9)	(6.0)
Income Tax Expense (Benefit)	(2.7)	(3.2)
Net income	16.9	18.3
Nonoperating Income (Expense)	(6.2)	(6.1)
Accumulated other comprehensive income	(31.1)		(31.8)
Other Comprehensive Income (Loss), Net of Tax	0.7	1.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	(0.7)
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, after Tax	0.7	1.0
Income tax (expense)/benefit on reclassification of earnings related to pension and postretirement activity	(0.2)	(0.3)
Contract with Customer, Asset, before Allowance for Credit Loss	63.5		$ 60.8
Subsidiaries [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Significant Accounting Policies [Line Items]
Net income	0.7	1.0
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)		(0.3)
Net income	0.7	1.0
Nonoperating Income (Expense)	$ 0.9	$ 1.3