Regulatory Matters (Details) - USD ($) $ in Millions					1 Months Ended	9 Months Ended	12 Months Ended
	Jan. 22, 2019	Oct. 01, 2018	Oct. 20, 2017	Jan. 30, 2017	Dec. 31, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Aug. 01, 2018	Dec. 31, 2017
Regulatory assets earning a return					$ 5.5			$ 5.5
Distribution Modernization Rider Period			3 years
DRO revenue requirement		$ 248.0
Regulatory Assets, Noncurrent					152.6			152.6		$ 163.2
Distribution Modernization Plan Cost of capital projects					576.0
Regulatory Assets, Current					41.1			41.1		23.9
Regulatory Assets					193.7			193.7		187.1
Regulatory Liability, Noncurrent					278.3			278.3		221.2
Regulatory Liability, Current					34.9			34.9		14.8
Regulatory Liabilities					313.2			313.2		236.0
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 29.8
Public Utilities, Approved Return on Equity, Percentage		9.999%
Public Utilities, Approved Debt Capital Structure, Percentage		4.80%
Distribution Investment Rider		$ 12.2
One-time Decoupling Rider Revenue Requirement						$ 3.7
TCJA Yearly Refund to Customers Per DRO		4.0
DRO Vegetation Management Cost Baseline								10.7
DRO Vegetation Management Cost Deferral Cap								$ 4.6
Distribution Modernization Rider			$ 105.0
Return on Equity SEET Threshold								12.00%
Proposed annual reduction in transmission rates to reflect effects of TCJA								$ 2.4
Due from PJM transmission settlement					16.5			16.5	$ 41.6	0.0
PJM transmission enhancement repayment amount								$ 14.3
Tax rate after Tax Cuts and Jobs Act of 2017								21.00%
Vegetation Management and Other [Member]
Regulatory Assets, Noncurrent					7.8			$ 7.8
Undercollections to be collected [Member]
Regulatory Assets Type of Recovery								A/B
Regulatory Current Asset, End Date for Recovery								2019
Regulatory Assets, Current					40.5			$ 40.5		23.9
Amounts being recovered through base rates [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								2019
Regulatory Assets, Current					0.6			$ 0.6		0.0
Pension Costs [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								Ongoing
Regulatory Assets, Noncurrent					87.5			$ 87.5		92.4
Unrecovered OVEC Charges [Member]
Regulatory Assets Type of Recovery								C
Regulatory Current Asset, End Date for Recovery								Undetermined
Regulatory Assets, Noncurrent					28.7			$ 28.7		27.8
Deferred Fuel Costs [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								2020
Regulatory Assets, Noncurrent					3.3			$ 3.3		9.3
Deferred Regulatory Compliance Costs [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								2020
Regulatory Assets, Noncurrent					6.1			$ 6.1		9.2
Ccem Smart Grid And Advanced Metering Infrastructure Cost [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								Undetermined
Regulatory Assets, Noncurrent					8.5			$ 8.5		7.3
Loss on Reacquired Debt [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								Various
Regulatory Assets, Noncurrent					6.0			$ 6.0		7.0
Storm Costs [Member]
Regulatory Assets Type of Recovery								A
Regulatory Current Asset, End Date for Recovery								Undetermined
Regulatory Assets, Noncurrent					4.7			$ 4.7		2.1
Vegetation Management [Member]
Regulatory Assets Type of Recovery								A/B
Regulatory Current Asset, End Date for Recovery								Undetermined
Regulatory Assets, Noncurrent					4.6			$ 4.6		8.1
Subsidiaries [Member]
Regulatory assets earning a return					5.5			5.5
Distribution Modernization Rider Period				3 years
DRO revenue requirement		248.0
Regulatory Assets, Noncurrent					152.6			152.6		163.2
Distribution Modernization Plan Cost of capital projects					576.0
Regulatory Assets, Current					41.1			41.1		23.9
Regulatory Assets					193.7			193.7		187.1
Regulatory Liability, Noncurrent					278.3			278.3		221.2
Regulatory Liability, Current					34.9			34.9		14.8
Regulatory Liabilities					313.2			313.2		236.0
Public Utilities, Approved Rate Increase (Decrease), Amount		$ 29.8
Public Utilities, Approved Return on Equity, Percentage		9.999%
Public Utilities, Approved Debt Capital Structure, Percentage		4.80%
Distribution Investment Rider		$ 12.2
One-time Decoupling Rider Revenue Requirement						$ 3.7
TCJA Yearly Refund to Customers Per DRO		$ 4.0
DRO Vegetation Management Cost Deferral Cap								$ 4.6
Distribution Modernization Rider				$ 105.0
Return on Equity SEET Threshold								12.00%
Proposed annual reduction in transmission rates to reflect effects of TCJA								$ 2.4
Due from PJM transmission settlement					16.5			16.5	$ 41.6	0.0
PJM transmission enhancement repayment amount								$ 14.3
Tax rate after Tax Cuts and Jobs Act of 2017								21.00%
Subsidiaries [Member] | Vegetation Management and Other [Member]
Regulatory Assets, Noncurrent					7.8			$ 7.8
Subsidiaries [Member] | Undercollections to be collected [Member]
Regulatory Assets Type of Recovery								A/B
Regulatory Current Asset, End Date for Recovery								2019
Regulatory Assets, Current					40.5			$ 40.5		23.9
Subsidiaries [Member] | Amounts being recovered through base rates [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								2019
Regulatory Assets, Current					0.6			$ 0.6		0.0
Subsidiaries [Member] | Pension Costs [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								Ongoing
Regulatory Assets, Noncurrent					87.5			$ 87.5		92.4
Subsidiaries [Member] | Unrecovered OVEC Charges [Member]
Regulatory Assets Type of Recovery								C
Regulatory Current Asset, End Date for Recovery								Undetermined
Regulatory Assets, Noncurrent					28.7			$ 28.7		27.8
Subsidiaries [Member] | Deferred Fuel Costs [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								2020
Regulatory Assets, Noncurrent					3.3			$ 3.3		9.3
Subsidiaries [Member] | Deferred Regulatory Compliance Costs [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								2020
Regulatory Assets, Noncurrent					6.1			$ 6.1		9.2
Subsidiaries [Member] | Ccem Smart Grid And Advanced Metering Infrastructure Cost [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								Undetermined
Regulatory Assets, Noncurrent					8.5			$ 8.5		7.3
Subsidiaries [Member] | Loss on Reacquired Debt [Member]
Regulatory Assets Type of Recovery								B
Regulatory Current Asset, End Date for Recovery								Various
Regulatory Assets, Noncurrent					6.0			$ 6.0		7.0
Subsidiaries [Member] | Storm Costs [Member]
Regulatory Assets Type of Recovery								A
Regulatory Current Asset, End Date for Recovery								Undetermined
Regulatory Assets, Noncurrent					4.7			$ 4.7		2.1
Subsidiaries [Member] | Vegetation Management [Member]
Regulatory Assets Type of Recovery								A/B
Regulatory Current Asset, End Date for Recovery								Undetermined
Regulatory Assets, Noncurrent					4.6			$ 4.6		8.1
Postretirement Benefit Costs [Member]
Regulatory Liabilities Type of Recovery								B
Regulatory Liability, Amortization Through								Ongoing
Regulatory Liability, Noncurrent					6.0			$ 6.0		5.0
Postretirement Benefit Costs [Member] | Subsidiaries [Member]
Regulatory Liabilities Type of Recovery								B
Regulatory Liability, Amortization Through								Ongoing
Regulatory Liability, Noncurrent					6.0			$ 6.0		5.0
PJM transmission enhancement settlement repayment amount [Member]
Regulatory Liabilities Type of Recovery								A
Regulatory Liability, Amortization Through								2025
Regulatory Liability, Noncurrent					16.9			$ 16.9		0.0
PJM transmission enhancement settlement repayment amount [Member] | Subsidiaries [Member]
Regulatory Liabilities Type of Recovery								A
Regulatory Liability, Amortization Through								2025
Regulatory Liability, Noncurrent					16.9			$ 16.9		0.0
Deferred Income Tax Charge [Member]
Regulatory Liability, Amortization Through								Various
Regulatory Liability, Noncurrent					116.3			$ 116.3		83.4
Deferred Income Tax Charge [Member] | Subsidiaries [Member]
Regulatory Liability, Amortization Through								Various
Regulatory Liability, Noncurrent					116.3			$ 116.3		83.4
Removal Costs [Member]
Regulatory Liability, Amortization Through								Not Applicable
Regulatory Liability, Noncurrent					139.1			$ 139.1		132.8
Removal Costs [Member] | Subsidiaries [Member]
Regulatory Liability, Amortization Through								Not Applicable
Regulatory Liability, Noncurrent					139.1			$ 139.1		132.8
Overcollection of costs to be refunded [Member]
Regulatory Liabilities Type of Recovery								A/B
Regulatory Liability, Amortization Through								2019
Regulatory Liability, Noncurrent					34.9			$ 34.9		14.8
Overcollection of costs to be refunded [Member] | Subsidiaries [Member]
Regulatory Liabilities Type of Recovery								A/B
Regulatory Liability, Amortization Through								2018
Regulatory Liability, Noncurrent					34.9			$ 34.9		$ 14.8
Scenario, Forecast [Member]
Distribution Investment Rider							$ 22.0
DRO Vegetation Management Cost Baseline							15.7
DRO Vegetation Management Cost Deferral Cap							4.6
Requested Distribution Modernization Rider annual revenue through filing	$ 199.0
Scenario, Forecast [Member] | Subsidiaries [Member]
Distribution Investment Rider							22.0
DRO Vegetation Management Cost Baseline							15.7
Requested Distribution Modernization Rider annual revenue through filing	$ 199.0
Subsequent Event [Member] | Subsidiaries [Member]
DRO Vegetation Management Cost Deferral Cap							$ 4.6
Long-term [Member]
Due from PJM transmission settlement					10.8			10.8
Long-term [Member] | Subsidiaries [Member]
Due from PJM transmission settlement					10.8			10.8
Short-term [Member]
Due from PJM transmission settlement					16.5			16.5
Short-term [Member] | Subsidiaries [Member]
Due from PJM transmission settlement					$ 16.5			$ 16.5