Subsequent Events (Details)	Feb. 17, 2017 USD ($) Natural_gas_plant	Feb. 16, 2017 USD ($) agreement	Jan. 18, 2017 USD ($) Natural_gas_plant limited_guaranty MW	Dec. 06, 2016 USD ($)	Dec. 31, 2016 MW	Dec. 05, 2016 USD ($)
Purchase Agreement with Aspen Generating, LLC
Subsequent Event [Line Items]
Plant generation capacity (in MW's)					1,572
Subsequent Event | Purchase Agreement with Aspen Generating, LLC
Subsequent Event [Line Items]
Plant generation capacity (in MW's)			1,572
Cash purchase price | $	$ 925,000,000		$ 925,000,000
Number of limited guaranties | limited_guaranty			2
Subsequent Event | Purchase Agreement with Aspen Generating, LLC | Minimum
Subsequent Event [Line Items]
Term of guaranties			1 year
Subsequent Event | Purchase Agreement with Aspen Generating, LLC | Maximum
Subsequent Event [Line Items]
Term of guaranties			3 years
Subsequent Event | Purchase Agreement with Aspen Generating, LLC | Bath County Hydro
Subsequent Event [Line Items]
Plant generation capacity (in MW's)			713
Subsequent Event | Purchase Agreement with Aspen Generating, LLC | Springdale Generating Facility Units 1-5
Subsequent Event [Line Items]
Plant generation capacity (in MW's)			638
Subsequent Event | Purchase Agreement with Aspen Generating, LLC | Chamberburg Generating Facility Units 12-13
Subsequent Event [Line Items]
Plant generation capacity (in MW's)			88
Subsequent Event | Purchase Agreement with Aspen Generating, LLC | Gans Generating Facility Units 8-9
Subsequent Event [Line Items]
Plant generation capacity (in MW's)			88
Subsequent Event | Purchase Agreement with Aspen Generating, LLC | Hunlock Creek
Subsequent Event [Line Items]
Plant generation capacity (in MW's)			45
Subsequent Event | Purchase Agreement with Aspen Generating, LLC | AE Supply
Subsequent Event [Line Items]
Number of gas generating plants | Natural_gas_plant	4		4
Discharge of note indenture | $			$ 305,000,000
Make-whole premiums | $			$ 100,000,000
Subsequent Event | Purchase Agreement with Aspen Generating, LLC | AGC
Subsequent Event [Line Items]
Plant ownership percentage			59.00%
Term Loan
Subsequent Event [Line Items]
Face amount of loan | $				$ 1,200,000,000.0		$ 1,000,000,000
Debt Instrument, Term				5 years
Term Loan | Subsequent Event
Subsequent Event [Line Items]
Debt Instrument, Number of Agreements | agreement		2
Bank of America N.A. | $125M Term Loan | Term Loan | Subsequent Event
Subsequent Event [Line Items]
Face amount of loan | $		$ 125,000,000
Debt Instrument, Term		3 years
The Bank of Nova Scotia | $125M Term Loan | Term Loan | Subsequent Event
Subsequent Event [Line Items]
Face amount of loan | $		$ 125,000,000
Debt Instrument, Term		3 years