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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% |  |  |  |  |  |  |  |  |  |

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