Decommissioning Trust Funds (Details) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Securities held
Fair Value	$ 3,566.0	$ 3,596.0
Total Unrealized Gains	392	503
Total Unrealized Losses	45	21
Equity securities [Member]
Securities held
Fair Value	1,930.0	2,076.0
Total Unrealized Gains	277	436
Total Unrealized Losses	40	9
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months Fair Value	393	15
More than 12 months Fair Value	39	105
Total Fair Value	432	120
Less than 12 Months Gross Unrealized Losses	30	1
More than 12 Months Gross Unrealized Losses	10	8
Total Gross Unrealized Losses	40	9
Debt Securities [Member]
Securities held
Fair Value	1,636.0	1,520.0
Total Unrealized Gains	115	67
Total Unrealized Losses	5	12
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months Fair Value	166	474
More than 12 months Fair Value	54	4
Total Fair Value	220	478
Less than 12 Months Gross Unrealized Losses	3	11
More than 12 Months Gross Unrealized Losses	2	1
Total Gross Unrealized Losses	$ 5	$ 12

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