Regulatory Matters - Pennsylvania and West Virginia (Details) $ in Millions							1 Months Ended	3 Months Ended	12 Months Ended
	Mar. 09, 2018 USD ($)	Sep. 01, 2017 USD ($)	Jun. 14, 2017 USD ($)	Jan. 19, 2017 USD ($)	Dec. 09, 2016 USD ($)	Jun. 19, 2015	Mar. 31, 2016 USD ($)	Mar. 31, 2018 proposal	Dec. 31, 2019 USD ($)	Dec. 31, 2017 USD ($)	Mar. 06, 2017 USD ($) MW	Sep. 23, 2016 program
Pennsylvania | DSP June 2017- May 2019
Regulatory Matters [Line Items]
Number of RFP's | proposal								1
RFP term								2 years
Pennsylvania | DSP June 2019- May 2023
Regulatory Matters [Line Items]
Number of RFP's | proposal								2
RFP term								2 years
Pennsylvania | Three Month Period | DSP June 2019- May 2023
Regulatory Matters [Line Items]
Term of energy contract								3 months
Pennsylvania | Twelve month period | DSP June 2017- May 2019
Regulatory Matters [Line Items]
Term of energy contract								12 months
Pennsylvania | Twelve month period | DSP June 2019- May 2023
Regulatory Matters [Line Items]
Term of energy contract								12 months
Pennsylvania | Twenty-four month period | DSP June 2017- May 2019
Regulatory Matters [Line Items]
Term of energy contract								24 months
Pennsylvania | Twenty-four month period | DSP June 2019- May 2023
Regulatory Matters [Line Items]
Term of energy contract								24 months
Pennsylvania | Pennsylvania Companies | PPUC | EE&C Phase III
Regulatory Matters [Line Items]
Approved amount of annual increase							$ 390.0
Pennsylvania | ME
Regulatory Matters [Line Items]
Approved amount of annual increase				$ 96.0
Pennsylvania | ME | PPUC
Regulatory Matters [Line Items]
Demand reduction targets proposed						1.80%
Energy consumption reduction targets proposed						4.00%
Amount of requested rate increase (decrease)			$ (51.3)
Net annual effect on rates	$ 37.0
Pennsylvania | Penn
Regulatory Matters [Line Items]
Approved amount of annual increase				29.0
Pennsylvania | Penn | PPUC
Regulatory Matters [Line Items]
Demand reduction targets proposed						1.70%
Energy consumption reduction targets proposed						3.30%
Amount of requested rate increase (decrease)			(33.2)
Net annual effect on rates	9.0
Pennsylvania | WP
Regulatory Matters [Line Items]
Approved amount of annual increase				66.0
Pennsylvania | WP | PPUC
Regulatory Matters [Line Items]
Demand reduction targets proposed						1.80%
Energy consumption reduction targets proposed						2.60%
Amount of requested rate increase (decrease)			(50.1)
Net annual effect on rates	30.0
Pennsylvania | PN
Regulatory Matters [Line Items]
Approved amount of annual increase				$ 100.0
Pennsylvania | PN | PPUC
Regulatory Matters [Line Items]
Demand reduction targets proposed						0.00%
Energy consumption reduction targets proposed						3.90%
Amount of requested rate increase (decrease)			$ (44.8)
Net annual effect on rates	$ 40.0
West Virginia | WVPSC | VMP Surcharge Rates
Regulatory Matters [Line Items]
Amount of requested rate increase (decrease)		$ (15.0)
Requested rate decrease		1.00%
West Virginia | MP and PE | WVPSC
Regulatory Matters [Line Items]
Number of proposed efficient programs | program												3
Energy efficient reduction requirement (percent)												0.50%
Expenditures for cost recovery program										$ 10.4
West Virginia | MP and PE | WVPSC | ENEC
Regulatory Matters [Line Items]
Amount of requested rate increase (decrease)					$ (25.0)
Rate stabilization period					2 years
Scenario, Forecast | West Virginia | WVPSC | VMP Surcharge Rates
Regulatory Matters [Line Items]
Amount of additional requested rate increase (decrease)									$ 15.0
Pleasants Power Station
Regulatory Matters [Line Items]
Plant capacity (in MW's) | MW											1,300
Consideration transferred											$ 195.0