Derivatives (Volumes Of Outstanding Derivative Contracts, In Megawatt Hours Unless Otherwise Specified) (Details)		Sep. 30, 2017	Dec. 31, 2016
Forwards And Swaps [Member] | Natural Gas [Member]
Derivative [Line Items]
Derivative Number Of Instruments Held	[1],[2]	300,594,593	323,301,331
Forwards And Swaps [Member] | Electricity [Member]
Derivative [Line Items]
Derivative Number Of Instruments Held		3,505,504	3,287,397
Options [Member] | Natural Gas [Member]
Derivative [Line Items]
Derivative Number Of Instruments Held	[1],[2]	79,640,435	96,602,785
Congestion Revenue Rights [Member] | Electricity [Member]
Derivative [Line Items]
Derivative Number Of Instruments Held	[3]	249,876,873	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.