Decommissioning Trust Funds (Details) (USD $) In Millions	6 Months Ended	12 Months Ended
Jun. 30, 2011	Dec. 31, 2010
Securities held
Fair Value	$ 3,775.0	$ 3,596.0
Total Unrealized Gains	600	503
Total Unrealized Losses	10	21
Equity securities [Member]
Securities held
Fair Value	2,204.0	2,076.0
Total Unrealized Gains	525	436
Total Unrealized Losses	5	9
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months Fair Value	49	15
More than 12 months Fair Value	52	105
Total Fair Value	101	120
Less than 12 Months Gross Unrealized Losses	1	1
More than 12 Months Gross Unrealized Losses	4	8
Total Gross Unrealized Losses	5	9
Debt Securities [Member]
Securities held
Fair Value	1,571.0	1,520.0
Total Unrealized Gains	75	67
Total Unrealized Losses	5	12
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value
Less than 12 months Fair Value	314	474
More than 12 months Fair Value	5	4
Total Fair Value	319	478
Less than 12 Months Gross Unrealized Losses	5	11
More than 12 Months Gross Unrealized Losses		1
Total Gross Unrealized Losses	$ 5	$ 12

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading. Such securities are reported at fair value, with unrealized gains and losses excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale Security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain or loss of an Available-for-sale Security that is designated as being hedged in a fair value hedge shall be recognized in earnings during the period of the hedge, as should other than temporary declines in fair value below costs basis.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 16

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 22

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 12
-Subparagraph b

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 13

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 115
-Paragraph 14

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This item represents the excess of [amortized] cost over fair value of securities that have been in a loss position for twelve months or longer for those securities which are categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Emerging Issues Task Force (EITF)
-Number 03-1
-Paragraph 21
-Subparagraph a(2)

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name FASB Staff Position (FSP)
-Number FAS115-1/124-1
-Paragraph 17
-Subparagraph a(1)

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This item represents the excess of [amortized] cost over fair value of securities in a loss position and which are categorized neither as held-to-maturity nor trading securities.

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 133
-Paragraph 22

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
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