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Regulatory Matters - Pennsylvania and West Virginia (Details) $ in Thousands						1 Months Ended	4 Months Ended	6 Months Ended	12 Months Ended
	Jan. 18, 2019 USD ($)	Aug. 31, 2018 USD ($)	May 30, 2018 USD ($)	Mar. 09, 2018 USD ($)	Dec. 11, 2017 proposal MW	Mar. 31, 2016 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 proposal	Mar. 06, 2017 MW	Sep. 30, 2016 program
Pennsylvania
Regulatory Matters [Line Items]
Project term									2 years
Number of RFP's \| proposal									1
Pennsylvania \| 12 Month Period
Regulatory Matters [Line Items]
Energy contract term									12 months
Pennsylvania \| 24 Month Period
Regulatory Matters [Line Items]
Energy contract term									24 months
Pennsylvania \| PPUC \| Temporary Rates Order Tracked Value
Regulatory Matters [Line Items]
Rate filing period									3 years
Pennsylvania \| PPUC \| ME
Regulatory Matters [Line Items]
Annual effect				$ 37,000
Pennsylvania \| PPUC \| ME \| Temporary Rates Order Tracked Value
Regulatory Matters [Line Items]
Approved amount of rate increase							$ 12,000
Pennsylvania \| PPUC \| ME \| Voluntary Surcharges
Regulatory Matters [Line Items]
Approved amount of rate increase								$ (22,000)
Pennsylvania \| PPUC \| Penn
Regulatory Matters [Line Items]
Annual effect				9,000
Pennsylvania \| PPUC \| Penn \| Temporary Rates Order Tracked Value
Regulatory Matters [Line Items]
Approved amount of rate increase							3,000
Pennsylvania \| PPUC \| Penn \| Voluntary Surcharges
Regulatory Matters [Line Items]
Approved amount of rate increase								(6,000)
Pennsylvania \| PPUC \| WP
Regulatory Matters [Line Items]
Annual effect				30,000
Pennsylvania \| PPUC \| WP \| Temporary Rates Order Tracked Value
Regulatory Matters [Line Items]
Approved amount of rate increase							10,000
Pennsylvania \| PPUC \| WP \| Voluntary Surcharges
Regulatory Matters [Line Items]
Approved amount of rate increase								(18,000)
Pennsylvania \| PPUC \| Pennsylvania Companies \| DSP
Regulatory Matters [Line Items]
Project term					2 years
Number of RFP's \| proposal					2
New hourly priced default service threshold (in MW's) \| MW					0.1
Pennsylvania \| PPUC \| Pennsylvania Companies \| DSP \| 3 Month Period
Regulatory Matters [Line Items]
Energy contract term					3 months
Pennsylvania \| PPUC \| Pennsylvania Companies \| DSP \| 12 Month Period
Regulatory Matters [Line Items]
Energy contract term					12 months
Pennsylvania \| PPUC \| Pennsylvania Companies \| DSP \| 24 Month Period
Regulatory Matters [Line Items]
Energy contract term					24 months
Pennsylvania \| PPUC \| Pennsylvania Companies \| EE&C
Regulatory Matters [Line Items]
Approved amount of rate increase						$ (390,000)
Pennsylvania \| PPUC \| PN
Regulatory Matters [Line Items]
Annual effect				$ 40,000
Pennsylvania \| PPUC \| PN \| Temporary Rates Order Tracked Value
Regulatory Matters [Line Items]
Approved amount of rate increase							$ 13,000
Pennsylvania \| PPUC \| PN \| Voluntary Surcharges
Regulatory Matters [Line Items]
Approved amount of rate increase								$ (23,000)
West Virginia \| WVPSC \| MP and PE
Regulatory Matters [Line Items]
Number of proposed efficient programs \| program											3
Energy efficient reduction requirement (percent)											0.50%
Annual effect			$ 26,200
West Virginia \| WVPSC \| MP and PE \| ENEC
Regulatory Matters [Line Items]
Requested rate increase (decrease)		$ 100,900
Annual effect		25,600
West Virginia \| WVPSC \| MP and PE \| Elimination of Energy Efficiency Cost Rate Surcharge
Regulatory Matters [Line Items]
Requested rate increase (decrease)		$ 2,100
Requested rate increase (decrease) (percent)		7.20%
Pleasants Power Station
Regulatory Matters [Line Items]
Plant generation capacity (in MW's) \| MW										1,300
Subsequent Event \| Pennsylvania \| ME
Regulatory Matters [Line Items]
Requested rate increase (decrease)	$ 44,520
Subsequent Event \| Pennsylvania \| Penn
Regulatory Matters [Line Items]
Requested rate increase (decrease)	26,060
Subsequent Event \| Pennsylvania \| WP
Regulatory Matters [Line Items]
Requested rate increase (decrease)	50,850
Subsequent Event \| Pennsylvania \| PN
Regulatory Matters [Line Items]
Requested rate increase (decrease)	$ 24,720