Regulatory Matters - Pennsylvania and West Virginia (Details) $ in Thousands												6 Months Ended	12 Months Ended
	Dec. 09, 2016 USD ($)	Nov. 21, 2016 brief	Oct. 14, 2016 USD ($) proceeding Party	Sep. 28, 2016 USD ($)	Aug. 22, 2016 USD ($)	Aug. 16, 2016 USD ($)	Aug. 05, 2016 MW	Apr. 28, 2016 USD ($)	Nov. 03, 2015 proposal	Oct. 19, 2015 USD ($)	Jun. 19, 2015	Jun. 30, 2016 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) proposal program	Dec. 31, 2027 MW	Dec. 31, 2020 MW	Feb. 03, 2017 Party	Dec. 01, 2016 Party
Pennsylvania
Regulatory Matters [Line Items]
Project term									2 years					2 years
Number of requests for proposal | proposal									1,000,000					1
Pennsylvania | 3 Month Period
Regulatory Matters [Line Items]
Energy contract term														3 months
Pennsylvania | 12 Month Period
Regulatory Matters [Line Items]
Energy contract term									12 months					12 months
Pennsylvania | 24 Month Period
Regulatory Matters [Line Items]
Energy contract term									24 months					24 months
Pennsylvania | Unfavorable Regulatory Action
Regulatory Matters [Line Items]
Potential cost of compliance														$ 174,000
Pennsylvania | ME
Regulatory Matters [Line Items]
Requested rate increase (decrease)								$ 140,200
Interim amount of rate increase (decrease)			$ 96,000
Pennsylvania | Penn
Regulatory Matters [Line Items]
Requested rate increase (decrease)								42,000
Interim amount of rate increase (decrease)			29,000
Pennsylvania | WP
Regulatory Matters [Line Items]
Requested rate increase (decrease)								98,200
Interim amount of rate increase (decrease)			66,000
Pennsylvania | PN
Regulatory Matters [Line Items]
Requested rate increase (decrease)								$ 158,800
Interim amount of rate increase (decrease)			$ 100,000
Pennsylvania | PPUC
Regulatory Matters [Line Items]
LTIP recovery period										5 years
Pennsylvania | PPUC | DSIC
Regulatory Matters [Line Items]
Loss Contingency, Pending Proceedings, Number Consolidated | proceeding			4
Pennsylvania | PPUC | DSIC Rates
Regulatory Matters [Line Items]
Public Utilities, Number of Parties Who Filed A Brief | Party			2															1
Loss Contingency, Claims Dismissed, Number | brief		1
Pennsylvania | PPUC | ME
Regulatory Matters [Line Items]
Proposed ROE (percent)											1.80%
Energy consumption reduction targets (percent)											4.00%
Requested rate increase (decrease)										$ 43,440
Pennsylvania | PPUC | Penn
Regulatory Matters [Line Items]
Proposed ROE (percent)											1.70%
Energy consumption reduction targets (percent)											3.30%
Requested rate increase (decrease)										56,350
Pennsylvania | PPUC | WP
Regulatory Matters [Line Items]
Proposed ROE (percent)											1.80%
Energy consumption reduction targets (percent)											2.60%
Requested rate increase (decrease)										88,340
Pennsylvania | PPUC | PN
Regulatory Matters [Line Items]
Proposed ROE (percent)											0.00%
Energy consumption reduction targets (percent)											3.90%
Requested rate increase (decrease)										$ 56,740
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														$ 10,400
Minimum energy requirement to file and implement a RFP (in MW's) | MW							100
Actual under-recovery balance												$ 119,000
West Virginia | WVPSC | MP and PE | ENEC
Regulatory Matters [Line Items]
Requested rate increase (decrease)						$ 65,000
Requested rate increase (decrease) (percent)						4.70%
Approved amount of rate increase	$ 25,000
Rate stability period	2 years
West Virginia | WVPSC | MP and PE | ENEC | Forecast
Regulatory Matters [Line Items]
Projected over-recovery amount													$ 54,000
West Virginia | WVPSC | MP and PE | Modernization and Improvement Plan for Coal-Fired Boilers
Regulatory Matters [Line Items]
Requested rate increase (decrease)					$ 6,900
Requested rate increase (decrease) (percent)					0.50%
West Virginia | WVPSC | MP and PE | Accelerated Recovery Costs For Modernizing and Improving Coal-Fired Boilers
Regulatory Matters [Line Items]
Requested rate increase (decrease)				$ 7,400
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
Subsequent Event | Pennsylvania | PPUC | DSIC Rates
Regulatory Matters [Line Items]
Public Utilities, Number of Parties Who Filed A Brief | Party																	1