Supplemental Financial Information (Supplemental Financial Information) (Details) - USD ($) $ in Millions	3 Months Ended					9 Months Ended
	Sep. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Supplemental Financial Information [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						$ 0.0
Accounts Receivable, Allowance for Credit Loss, Current	$ 3.2					3.2		$ 0.4
Customer receivables	52.2					52.2		45.7
Unbilled Revenue	15.1					15.1		19.4
Amounts due from partners in jointly owned stations	0.2					0.2		0.3
Due from PJM transmission settlement	1.8					1.8		1.8
Other	2.1					2.1		1.1
Provision for uncollectible accounts	(3.2)					(3.2)		(0.4)
Total accounts receivable, net	68.2					68.2		67.9
Total inventories, at average cost	8.2					8.2		10.4
Other Operating Income (Expense), Net	(0.1)		$ 0.0			(0.1)	$ 0.0
Accounts Receivable, Credit Loss Expense (Reversal)						2.7
Accounts Receivable, Allowance for Credit Loss, Writeoff						(3.5)
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						0.0
Other Comprehensive Income (Loss), Net of Tax	0.1	$ (0.3)	(1.0)		$ (0.4)	0.0	(2.2)
Accounts Receivable, Allowance for Credit Loss, Recovery						3.6
Subsidiaries [Member]
Supplemental Financial Information [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.1
Accounts Receivable, Allowance for Credit Loss, Current	3.2					3.2		0.4
Customer receivables	51.2					51.2		45.0
Unbilled Revenue	15.1					15.1		19.4
Amounts due from partners in jointly owned stations	2.0					2.0		3.9
Due from PJM transmission settlement	1.8					1.8		1.8
Other	2.3					2.3		1.2
Provision for uncollectible accounts	(3.2)					(3.2)		(0.4)
Total accounts receivable, net	69.2					69.2		70.9
Total inventories, at average cost	8.2					8.2		$ 10.4
Accounts Receivable, Credit Loss Expense (Reversal)						2.7
Accounts Receivable, Allowance for Credit Loss, Writeoff						3.5
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						(2.1)
Other Comprehensive Income (Loss), Net of Tax	$ 0.9		$ 1.6	$ (0.9)	$ 0.4	2.2	$ 1.1
Amortization of defined benefit pension items [Member]
Supplemental Financial Information [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						(0.8)
Other Comprehensive Income (Loss), Net of Tax						0.8
Amortization of defined benefit pension items [Member] | Subsidiaries [Member]
Supplemental Financial Information [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						(2.4)
Other Comprehensive Income (Loss), Net of Tax						2.4
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member]
Supplemental Financial Information [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.0
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						0.8
Other Comprehensive Income (Loss), Net of Tax						(0.8)
Accumulated Net Gain (Loss) from Cash Flow Hedges Attributable to Parent [Member] | Subsidiaries [Member]
Supplemental Financial Information [Line Items]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax						0.1
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent						0.3
Other Comprehensive Income (Loss), Net of Tax						$ (0.2)