Derivatives (Volumes Of Outstanding Derivative Contracts, In Megawatt Hours Unless Otherwise Specified) (Detail)	3 Months Ended		12 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Derivative [Line Items]
Derivatives expiration, lower (year)	2018 years		2018 years
Derivatives expiration, higher (year)	2023 years		2023 years
Forwards And Swaps [Member] | Natural Gas [Member]
Derivative [Line Items]
Less Than 1 Year	311,804,316	[1],[2],[3]	329,466,510	[1],[2],[3]
Greater Than 1 Year but Less Than 3 Years	85,857,500	[1],[2],[3]	98,628,398	[1],[2],[3]
Greater Than 3 Years but Less Than 5 Years	4,812,500	[1],[2],[3]	5,490,000	[1],[2],[3]
Greater Than 5 Years	0	[1],[2],[3],[4]	0	[1],[2],[3],[4]
Forwards And Swaps [Member] | Electricity [Member]
Derivative [Line Items]
Less Than 1 Year	2,537,023	[1]	2,537,023	[1]
Greater Than 1 Year but Less Than 3 Years	3,164,680	[1]	3,541,046	[1]
Greater Than 3 Years but Less Than 5 Years	2,008,046	[1]	2,009,505	[1]
Greater Than 5 Years	2,402,346	[1],[4]	2,538,718	[1],[4]
Options [Member] | Natural Gas [Member]
Derivative [Line Items]
Less Than 1 Year	209,274,282	[1],[2],[3]	221,587,431	[1],[2],[3]
Greater Than 1 Year but Less Than 3 Years	166,356,071	[1],[2],[3]	216,279,767	[1],[2],[3]
Greater Than 3 Years but Less Than 5 Years	7,050,000	[1],[2],[3]	10,050,000	[1],[2],[3]
Greater Than 5 Years	0	[1],[2],[3],[4]	0	[1],[2],[3],[4]
Options [Member] | Electricity [Member]
Derivative [Line Items]
Less Than 1 Year	21,002	[1]	0	[1]
Greater Than 1 Year but Less Than 3 Years	239,233	[1]	239,015	[1]
Greater Than 3 Years but Less Than 5 Years	239,015	[1]	239,233	[1]
Greater Than 5 Years	98,505	[1],[4]	119,508	[1],[4]
Congestion Revenue Rights [Member] | Electricity [Member]
Derivative [Line Items]
Less Than 1 Year	63,826,023	[1]	74,198,690	[1]
Greater Than 1 Year but Less Than 3 Years	74,481,760	[1]	74,187,803	[1]
Greater Than 3 Years but Less Than 5 Years	74,358,484	[1]	74,240,147	[1]
Greater Than 5 Years	17,972,340	[1],[4]	25,699,804	[1],[4]

[1] Amounts shown reflect the total gross derivative volumes by commodity type that are expected to settle in each period.

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

[3] Million British Thermal Units.

[4] Derivatives in this category expire between 2018 and 2023.