Commitments and Contingencies (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended
	Sep. 30, 2011	Jun. 30, 2011	Jun. 30, 2011	Oct. 31, 2008
Emissions allowances			$ 57,000,000
Cost to install cooling towers			600,000,000
Remaining expenditure on installation			150,000,000
Exposure under guarantees		4,583,000,000	4,583,000,000
Estimate of possible loss		254,000,000	254,000,000	155,000,000
Damages claimed	155,000,000	64,000,000
Purchased surety bonds		120,000,000	120,000,000
Authorized issuance of standby letters of credit		55,000,000	55,000,000
Severance costs before tax		17,000,000
Severance costs		11,000,000
Property, Plant And Equipment [Member]
Legal settlements		24,000,000
Depreciation Expense [Member]
Legal settlements		7,000,000
Operations And Maintenance Expense [Member]
Legal settlements		17,000,000
Millstone Power Station [Member]
Estimate of possible loss				43,000,000
Minimum [Member]
Authorized civil penalties			25,000
Maximum [Member]
Authorized civil penalties			37,500
Third Party and Equity Method Investee [Member]
Issued guarantees		91,000,000	91,000,000
Exposure under guarantees		49,000,000	49,000,000
Virginia Power [Member]
Estimate of possible loss				$ 112,000,000