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Regulatory Matters - Reliability Matters and FERC (Details) $ in Millions											3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 19, 2018 proposal	May 31, 2018 USD ($)	May 14, 2018	Oct. 13, 2017	Mar. 10, 2017	Feb. 20, 2017	Jan. 19, 2017	Jun. 15, 2016 kv	Aug. 04, 2014	Aug. 24, 2012 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016	Dec. 31, 2015	Apr. 10, 2018 condition
Regulatory Matters [Line Items]
Impairment of assets and related charges												$ 0.0	$ 131.0
FERC
Regulatory Matters [Line Items]
Power threshold | kv								500
Denied recovery charges of exit fees											$ 78.8	$ 78.8
Number of alternative proposals filed | condition																2
FERC | Clean MOPR Complaints
Regulatory Matters [Line Items]
Number of proposed alternatives | proposal	2
FERC | PJM Transmission Rates
Regulatory Matters [Line Items]
Load-ratio share														100.00%	50.00%
Solution-based distribution factor															50.00%
Gain related to settlement		$ 77.0
FERC | PATH-Allegheny
Regulatory Matters [Line Items]
Cost recovery, PP&E reclassified to Regulatory Assets										$ 62.0
FERC | Path-WV
Regulatory Matters [Line Items]
Cost recovery, PP&E reclassified to Regulatory Assets										$ 59.0
FERC | PATH
Regulatory Matters [Line Items]
Proposed return on equity										10.90%
Base return on equity						10.40%	8.11%			10.40%
Return on equity granted for regional transmission organization participation										0.50%
Remaining recovery period of regulatory assets										5 years
FERC | The Ohio Companies | ESP IV PPA
Regulatory Matters [Line Items]
Term of proposed purchase power agreement									8 years
FERC | MAIT
Regulatory Matters [Line Items]
Approved suspension period					5 months
Approved ROE				11.00%								10.30%
Approved equity capital structure				60.00%
Impairment of assets and related charges
FERC | JCP&L
Regulatory Matters [Line Items]
Approved suspension period					5 months
FERC | ME, PN and Penn
Regulatory Matters [Line Items]
Requested rate increase (percentage)			6.70%

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