Fair Value Measurements (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended		3 Months Ended			1 Months Ended		12 Months Ended	3 Months Ended				3 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended			12 Months Ended
	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2012 Kewaunee Utility	Jun. 30, 2012 Salem Harbor	Dec. 31, 2012 Fair Value, Measurements, Nonrecurring Kewaunee	Jun. 30, 2010 Fair Value, Measurements, Nonrecurring State Line	Mar. 31, 2011 Fair Value, Measurements, Nonrecurring State Line Other operations and maintenance expense	Jun. 30, 2010 Fair Value, Measurements, Nonrecurring State Line Other operations and maintenance expense	Dec. 31, 2011 Fair Value, Measurements, Nonrecurring Salem Harbor Other operations and maintenance expense	Sep. 30, 2012 Fair Value, Measurements, Nonrecurring Emissions allowances Other operations and maintenance expense	Mar. 31, 2011 Maximum Fair Value, Measurements, Nonrecurring State Line	Dec. 31, 2011 Maximum Fair Value, Measurements, Nonrecurring Salem Harbor	Sep. 30, 2012 Maximum Fair Value, Measurements, Nonrecurring Emissions allowances	Mar. 31, 2010 Virginia Electric and Power Company	Dec. 31, 2011 Virginia Electric and Power Company	Sep. 30, 2010 Virginia Electric and Power Company Fair Value, Measurements, Nonrecurring Emissions allowances Other operations and maintenance expense	Sep. 30, 2012 Virginia Electric and Power Company Fair Value, Measurements, Nonrecurring Emissions allowances Other operations and maintenance expense	Sep. 30, 2011 Virginia Electric and Power Company Maximum Fair Value, Measurements, Nonrecurring Emissions allowances	Dec. 31, 2012 Elwood	Dec. 31, 2012 Brayton Point Fair Value, Measurements, Nonrecurring	Dec. 31, 2012 Brayton Point Fair Value, Measurements, Nonrecurring Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Ownership percentage																			50.00%
Impairment charge		$ 2,100			$ 378															$ 1,600
Impairment and other charges, net of tax					281															1,000
Long lived assets estimated fair value																					216
Number of utilities			2
Impairment and other charges					435
Long-lived assets impairment charges, net of tax					244
Inventory write-down					33
Inventory write down, net of tax					21
Severance costs	206				24									123
Severance costs net of tax					16
Impairment charge							55	163	31						228
Other Asset Impairment Charges Net Of Tax							39	107	20
Estimated fair value						59					1	1
After tax charge on disposal of assets				27
Pretax charge on disposal of assets				16
Salem Harbor, impairment charge										57						13	43
Impairment charge after tax																8
So2 emissions allowances obtained for arp cnd cair compliance										57							43
Impairment of intangible assets after tax										34							26
Maximum fair value of So2 emissions allowances													$ 1					$ 1