Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Details)	Mar. 31, 2012		Dec. 31, 2011
Coal (In Tons) [Member]
Derivative [Line Items]
NPNS Contract	140,000,000	[1]	147,000,000	[1]
Cash Flow Hedges		[2],[3]		[2],[3]
Other Derivatives	4,000,000	[4]		[2],[4]
Derivatives That Qualify for Regulatory Deferral		[2],[5]		[5]
Fuel Oils (In Gallons) [Member]
Derivative [Line Items]
NPNS Contract		[1],[2],[6]		[1],[2],[6]
Cash Flow Hedges		[2],[3],[6]		[2],[3],[6]
Other Derivatives	51,000,000	[4],[6]	36,000,000	[4],[6]
Derivatives That Qualify for Regulatory Deferral	45,000,000	[5],[6]	53,000,000	[5],[6]
Natural Gas (In Mmbtu) [Member]
Derivative [Line Items]
NPNS Contract	41,000,000	[1]	50,000,000	[1]
Cash Flow Hedges		[2],[3]		[2],[3]
Other Derivatives	20,000,000	[4]	17,000,000	[4]
Derivatives That Qualify for Regulatory Deferral	185,000,000	[5]	193,000,000	[5]
Power (In Megawatt Hours) [Member]
Derivative [Line Items]
NPNS Contract	90,000,000	[1]	73,000,000	[1]
Cash Flow Hedges	19,000,000	[3]	17,000,000	[3]
Other Derivatives	44,000,000	[4]	31,000,000	[4]
Derivatives That Qualify for Regulatory Deferral	26,000,000	[5]	21,000,000	[5]
Uranium (In Pounds) [Member]
Derivative [Line Items]
NPNS Contract	5,553,000	[1]	5,553,000	[1]
Cash Flow Hedges		[2],[3]		[2],[3]
Other Derivatives		[2],[4]		[2],[4]
Derivatives That Qualify for Regulatory Deferral	148,000	[5]	148,000	[5]

[1] Contracts through December 2017, March 2015, September 2035, and October 2024 for coal, natural gas, power, and uranium, respectively, as of March 31, 2012.

[2] Not applicable.

[3] Contracts through December 2016 for power as of March 31, 2012.

[4] Contracts through December 2014, October 2015, January 2013, and November 2016 for coal, fuel oils, natural gas, and power, respectively, as of March 31, 2012.

[5] Contracts through October 2014, October 2016, May 2032, and December 2013 for fuel oils, natural gas, power, and uranium, respectively, as of March 31, 2012.

[6] Fuel oils consist of heating and crude oil.