Regulatory Matters - Reliability and FERC Matters (Details) $ in Millions							1 Months Ended	12 Months Ended
	Feb. 20, 2017	Feb. 17, 2017 USD ($) Natural_gas_plant	Jan. 19, 2017	Jan. 18, 2017 USD ($) Natural_gas_plant MW	Aug. 24, 2016 USD ($)	Aug. 24, 2012 USD ($)	Apr. 30, 2007 kv	Dec. 31, 2016 USD ($) entity MW
Regulatory Matters [Line Items]
Regional enforcement entities | entity								8
FERC
Regulatory Matters [Line Items]
Power threshold for cost methodology (in KW) | kv							500
Denied recovery charges of exit fees | $								$ 78.8
PATH-Allegheny | FERC
Regulatory Matters [Line Items]
Cost recovery PP&E reclassified to Regulatory Assets | $						$ 62.0
Path-WV | FERC
Regulatory Matters [Line Items]
Cost recovery PP&E reclassified to Regulatory Assets | $						$ 59.0
PATH | FERC
Regulatory Matters [Line Items]
Cost recovery proposed ROE (percent)						10.90%
Base ROE (percent)						10.40%
ROE granted for RTO's (percent)						0.50%
Remaining recovery period of regulatory assets						5 years
Settled Litigation | California Claims Matters | FERC
Regulatory Matters [Line Items]
Settlement amount | $					$ 3.6
Purchase Agreement with Aspen Generating, LLC
Regulatory Matters [Line Items]
Plant generation capacity (in MW's)								1,572
Subsequent Event | PATH | FERC
Regulatory Matters [Line Items]
Base ROE (percent)	10.40%		8.11%
Subsequent Event | Purchase Agreement with Aspen Generating, LLC
Regulatory Matters [Line Items]
Cash purchase price | $		$ 925.0		$ 925.0
Plant generation capacity (in MW's)				1,572
Subsequent Event | Purchase Agreement with Aspen Generating, LLC | AE Supply
Regulatory Matters [Line Items]
Number of gas generating plants | Natural_gas_plant		4		4
Springdale Generating Facility Units 1-5 | Subsequent Event | Purchase Agreement with Aspen Generating, LLC
Regulatory Matters [Line Items]
Plant generation capacity (in MW's)				638
Chamberburg Generating Facility Units 12-13 | Subsequent Event | Purchase Agreement with Aspen Generating, LLC
Regulatory Matters [Line Items]
Plant generation capacity (in MW's)				88
Gans Generating Facility Units 8-9 | Subsequent Event | Purchase Agreement with Aspen Generating, LLC
Regulatory Matters [Line Items]
Plant generation capacity (in MW's)				88
Hunlock Creek | Subsequent Event | Purchase Agreement with Aspen Generating, LLC
Regulatory Matters [Line Items]
Plant generation capacity (in MW's)				45
Bath County Hydro | Subsequent Event | Purchase Agreement with Aspen Generating, LLC
Regulatory Matters [Line Items]
Plant generation capacity (in MW's)				713