Derivatives And Hedging Activities (Volumes Of Outstanding Derivative Contracts) (Details)		Dec. 31, 2017 MMBTU MWh	Dec. 31, 2016 MMBTU MWh
Forwards And Swaps [Member] | Natural Gas (MMBtus)[Member]
Derivative [Line Items]
Derivative Number of Instruments Held | MMBTU	[1],[2]	228,768,745	323,301,331
Forwards And Swaps [Member] | Electricity (Megawatt-hours) [Member]
Derivative [Line Items]
Derivative Number of Instruments Held | MWh		2,872,013	3,287,397
Options [Member] | Natural Gas (MMBtus)[Member]
Derivative [Line Items]
Derivative Number of Instruments Held | MMBTU	[1],[2]	60,736,806	96,602,785
Congestion Revenue Rights [Member] | Electricity (Megawatt-hours) [Member]
Derivative [Line Items]
Derivative Number of Instruments Held | MWh	[3]	312,272,177	278,143,281

[1] Amounts shown are for the combined positions of the electric fuels and core gas supply portfolios.

[2] Million British Thermal Units.

[3] CRRs are financial instruments that enable the holders to manage variability in electric energy congestion charges due to transmission grid limitations.