Regulatory Matters - Reliability Matters and FERC (Details) - FERC $ in Millions									3 Months Ended
	Dec. 21, 2017 USD ($)	Oct. 13, 2017	Mar. 10, 2017	Feb. 20, 2017	Jan. 19, 2017	Jun. 15, 2016 kv	Aug. 04, 2014	Aug. 24, 2012 USD ($)	Mar. 31, 2018 USD ($)	Sep. 30, 2017 USD ($)	Apr. 10, 2018 condition
Regulatory Matters [Line Items]
Power threshold | kv						500
Denied recovery charges of exit fees									$ 78.8
Subsequent Event
Regulatory Matters [Line Items]
Number of alternative proposals filed | condition											2
PATH-Allegheny
Regulatory Matters [Line Items]
Cost recovery, PP&E reclassified to Regulatory Assets								$ 62.0
Path-WV
Regulatory Matters [Line Items]
Cost recovery, PP&E reclassified to Regulatory Assets								$ 59.0
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
The Ohio Companies | ESP IV PPA
Regulatory Matters [Line Items]
Term of proposed purchase power agreement							8 years
MAIT
Regulatory Matters [Line Items]
Approved suspension period			5 months
Approved ROE		10.30%							11.00%
Approved equity capital structure		60.00%
Impairment of assets and related charges
JCP&L
Regulatory Matters [Line Items]
Approved suspension period			5 months
JCP&L | NITS
Regulatory Matters [Line Items]
Annual revenue requirement	$ 135.0
JCP&L | PJM Tariff
Regulatory Matters [Line Items]
Annual revenue requirement	$ 20.0