Regulatory Matters - Reliability and FERC Matters (Details) $ in Millions									3 Months Ended		9 Months Ended	12 Months Ended
	Dec. 20, 2017 USD ($)	Oct. 13, 2017	Mar. 10, 2017	Feb. 20, 2017	Jan. 19, 2017	Jun. 15, 2016 kv	Aug. 04, 2014	Aug. 24, 2012 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2017	Dec. 31, 2017 USD ($) entity	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Regulatory Matters [Line Items]
Regional enforcement entities | entity												8
Impairment of assets and related charges (Note 2)												$ 2,406.0	$ 10,665.0	$ 42.0
FERC
Regulatory Matters [Line Items]
Power threshold for cost methodology (in KW) | kv						500
Denied recovery charges of exit fees									$ 78.8			$ 78.8
Ohio Companies | FERC | Unit Power Agreement [Member]
Regulatory Matters [Line Items]
Term of proposed purchase power agreement							8 years
MAIT
Regulatory Matters [Line Items]
Impairment of assets and related charges (Note 2)										$ 13.0
MAIT | FERC
Regulatory Matters [Line Items]
Approved ROE		10.30%									11.00%
Approved capital structure		60.00%
JCP&L | FERC
Regulatory Matters [Line Items]
Impairment of assets and related charges (Note 2)									$ 28.0
Approved suspension period			5 months
JCP&L | FERC | Network Integration Transmission Service
Regulatory Matters [Line Items]
Annual revenue requirement	$ 135.0
JCP&L | FERC | PJM Tariff
Regulatory Matters [Line Items]
Annual revenue requirement	$ 20.0
PATH-Allegheny | FERC
Regulatory Matters [Line Items]
Cost recovery PP&E reclassified to Regulatory Assets								$ 62.0
Path-WV | FERC
Regulatory Matters [Line Items]
Cost recovery PP&E reclassified to Regulatory Assets								$ 59.0
PATH | FERC
Regulatory Matters [Line Items]
Cost recovery proposed ROE (percent)								10.90%
Base ROE (percent)				10.40%	8.11%			10.40%
ROE granted for RTO's (percent)								0.50%
Remaining recovery period of regulatory assets								5 years