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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