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Regulatory Matters - Reliability Matters and FERC (Details)
$ in Millions
 1 Months Ended 3 Months Ended
Feb. 20, 2017
Feb. 17, 2017
USD ($)
Jan. 19, 2017
Sep. 14, 2015
Aug. 04, 2014
Aug. 24, 2012
USD ($)
Jan. 31, 2017
USD ($)
MW
Mar. 31, 2017
USD ($)
entity
condition
subsidiary
MW
Jan. 18, 2017
MW
Regulatory Matters [Line Items]                  
Regional enforcement entities | entity               8  
Purchase Agreement with Aspen Generating, LLC                  
Regulatory Matters [Line Items]                  
Cash purchase price | $   $ 925.0         $ 925.0    
Plant capacity (in MW's)             1,572 1,572  
Springdale Generating Facility Units 1-5 | Purchase Agreement with Aspen Generating, LLC                  
Regulatory Matters [Line Items]                  
Plant capacity (in MW's)                 638
Chamberburg Generating Facility Units 12-13 | Purchase Agreement with Aspen Generating, LLC                  
Regulatory Matters [Line Items]                  
Plant capacity (in MW's)                 88
Gans Generating Facility Units 8-9 | Purchase Agreement with Aspen Generating, LLC                  
Regulatory Matters [Line Items]                  
Plant capacity (in MW's)                 88
Hunlock Creek | Purchase Agreement with Aspen Generating, LLC                  
Regulatory Matters [Line Items]                  
Plant capacity (in MW's)                 45
Bath County Hydro | Purchase Agreement with Aspen Generating, LLC                  
Regulatory Matters [Line Items]                  
Plant capacity (in MW's)                 713
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