Derivatives and Hedging Activities (Volume of Outstanding Derivative Contracts) (Details)	Jun. 30, 2011
Derivatives expiration, lower	2016
Derivatives expiration, higher	2022
Forwards Futures Swaps [Member] | Natural Gas [Member]
Less Than 1 Year	334,357,146	[1],[2],[3]
Greater Than 1 Year but Less Than 3 Years	260,860,000	[1],[2],[3]
Greater Than 3 Years but Less Than 5 Years	11,140,000	[1],[2],[3]
Greater Than 5 Years		[1],[2],[3],[4]
Forwards Futures Swaps [Member] | Electricity [Member]
Less Than 1 Year	4,918,603	[3]
Greater Than 1 Year but Less Than 3 Years	6,089,912	[3]
Greater Than 3 Years but Less Than 5 Years	2,914,999	[3]
Greater Than 5 Years	4,316,760	[3],[4]
Options Held [Member] | Natural Gas [Member]
Less Than 1 Year	238,196,542	[2],[3]
Greater Than 1 Year but Less Than 3 Years	269,857,756	[2],[3]
Greater Than 3 Years but Less Than 5 Years	31,200,000	[2],[3]
Greater Than 5 Years		[2],[3],[4]
Options Held [Member] | Electricity [Member]
Less Than 1 Year	3,284	[3]
Greater Than 1 Year but Less Than 3 Years	64,584	[3]
Greater Than 3 Years but Less Than 5 Years	264,348	[3]
Greater Than 5 Years	331,560	[3],[4]
Congested Revenue Rights [Member] | Electricity [Member]
Less Than 1 Year	58,509,308	[3]
Greater Than 1 Year but Less Than 3 Years	72,571,815	[3]
Greater Than 3 Years but Less Than 5 Years	72,649,100	[3]
Greater Than 5 Years	80,367,091	[3],[4]

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

[2] Million British Thermal Units.

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

[4] Derivatives in this category expire between 2016 and 2022.