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Regulatory Matters - Reliability Matters and FERC (Details) $ in Millions |  |  |  |  | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | ---
Feb. 20, 2017 | Jan. 19, 2017 | Aug. 04, 2014 | Aug. 24, 2012 | Jun. 30, 2017 USD ($) entity condition subsidiary | Dec. 31, 2012 USD ($)
Regulatory Matters [Line Items] |  |  |  |  |  |
Regional enforcement entities | entity |  |  |  |  | 8 |
FERC |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |
Denied recovery charges of exit fees |  |  |  |  | $ 78.8 |
Number of conditions | condition |  |  |  |  | 1 |
Market-based rate authority renewal period |  |  |  |  | 3 years |
FERC | PATH-Allegheny | PATH Transmission Project |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |
Cost recovery, PP&E reclassified to Regulatory Assets |  |  |  |  |  | $ 62.0
FERC | Path-WV | PATH Transmission Project |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |
Cost recovery, PP&E reclassified to Regulatory Assets |  |  |  |  |  | $ 59.0
FERC | PATH |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |
Return on equity (percent) | 10.40% | 8.11% |  | 10.40% |  |
FERC | PATH | PATH Transmission Project |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |
Proposed return on equity |  |  |  |  |  | 10.90%
Requested return on equity (percent) |  |  |  |  |  | 10.40%
Return on equity granted for regional transmission organization participation |  |  |  |  |  | 0.50%
Remaining recovery period of regulatory assets |  |  |  |  |  | 5 years
FERC | FET |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |
Number of stand-alone transmission subsidiaries | subsidiary |  |  |  |  | 2 |
FERC | The Ohio Companies | ESP IV PPA |  |  |  |  |  |
Regulatory Matters [Line Items] |  |  |  |  |  |
Proposed Purchase Power Agreement, Term |  |  | 8 years |  |  |

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