Regulatory Matters - Reliability Matters and FERC (Details) - FERC $ in Millions							3 Months Ended		12 Months Ended
	May 14, 2018	Feb. 20, 2017	Jan. 19, 2017	Jun. 15, 2016 kv	Aug. 04, 2014	Aug. 24, 2012 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2016	Dec. 31, 2015	Apr. 10, 2018 condition
Regulatory Matters [Line Items]
Power threshold | kv				500
Denied recovery charges of exit fees							$ 78.8
Number of alternative proposals filed | condition											2
PJM Transmission Rates
Regulatory Matters [Line Items]
Load-ratio share									100.00%	50.00%
Solution-based distribution factor										50.00%
Gain related to settlement							$ 42.0	$ 73.0
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
ME, PN and Penn
Regulatory Matters [Line Items]
Requested rate increase (percentage)	6.70%