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Regulatory matters.

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Regulatory Matters - Pennsylvania and West Virginia (Details) $ in Millions							1 Months Ended	12 Months Ended
	Dec. 11, 2017 proposal MW	Sep. 01, 2017 USD ($)	Jun. 14, 2017 USD ($)	Jan. 19, 2017 USD ($)	Dec. 09, 2016 USD ($)	Jun. 19, 2015	Mar. 31, 2016 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2017 USD ($) proposal	Dec. 31, 2027 MW	Dec. 31, 2020 MW	Mar. 06, 2017 USD ($) MW	Sep. 23, 2016 program
Pennsylvania
Regulatory Matters [Line Items]
Number of RFP's \| proposal									1
Project term									2 years
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 \| ME
Regulatory Matters [Line Items]
Approved amount of rate increase				$ 96.0
Pennsylvania \| Penn
Regulatory Matters [Line Items]
Approved amount of rate increase				29.0
Pennsylvania \| WP
Regulatory Matters [Line Items]
Approved amount of rate increase				66.0
Pennsylvania \| PN
Regulatory Matters [Line Items]
Approved amount of rate increase				$ 100.0
Pennsylvania \| PPUC \| ME
Regulatory Matters [Line Items]
Demand reduction targets proposed						1.80%
Energy consumption reduction targets (percent)						4.00%
Requested rate increase (decrease)			$ 51.3
Pennsylvania \| PPUC \| Penn
Regulatory Matters [Line Items]
Demand reduction targets proposed						1.70%
Energy consumption reduction targets (percent)						3.30%
Requested rate increase (decrease)			33.2
Pennsylvania \| PPUC \| WP
Regulatory Matters [Line Items]
Demand reduction targets proposed						1.80%
Energy consumption reduction targets (percent)						2.60%
Requested rate increase (decrease)			50.1
Pennsylvania \| PPUC \| Pennsylvania Companies \| DSP
Regulatory Matters [Line Items]
Number of RFP's \| proposal	2
Project term	2 years
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							$ 390.0
Pennsylvania \| PPUC \| PN
Regulatory Matters [Line Items]
Demand reduction targets proposed						0.00%
Energy consumption reduction targets (percent)						3.90%
Requested rate increase (decrease)			$ 44.8
West Virginia \| WVPSC \| VMS Rates
Regulatory Matters [Line Items]
Requested rate increase (decrease)		$ (15.0)
Requested rate increase (decrease) (percent)		(1.00%)
West Virginia \| WVPSC \| VMS Rates \| Forecast
Regulatory Matters [Line Items]
Requested additional rate decrease								$ (15.0)
West Virginia \| WVPSC \| MP and PE
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 \| WVPSC \| MP and PE \| ENEC
Regulatory Matters [Line Items]
Requested rate increase (decrease)					$ 25.0
Rate stability period					2 years
West Virginia \| WVPSC \| MP \| Accelerated Recovery Costs For Modernizing and Improving Coal-Fired Boilers \| Forecast
Regulatory Matters [Line Items]
Capacity shortfall (in MW's) \| MW											700
West Virginia \| WVPSC \| MP \| IRP \| Forecast
Regulatory Matters [Line Items]
Capacity shortfall (in MW's) \| MW										850
Pleasants Power Station
Regulatory Matters [Line Items]
Plant generation capacity (in MW's) \| MW												1,300
Assets purchase agreement consideration to be received												$ 195.0