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Regulatory Matters - Reliability Matters and FERC (Details)
$ in Millions
 9 Months Ended
Aug. 24, 2016
USD ($)
Sep. 14, 2015
Aug. 24, 2012
USD ($)
Sep. 30, 2016
USD ($)
entities
subsidiary
Regulatory Matters [Line Items]        
Regional enforcement entities | entities       8
FERC        
Regulatory Matters [Line Items]        
Denied recovery charges of exit fees       $ 78.8
FERC | California Claims Matters | Settled Litigation        
Regulatory Matters [Line Items]        
Settlement amount $ 3.6      
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 | 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