Risk Management and Fair Values (Details 2) (USD $) In Millions	Sep. 30, 2011	Dec. 31, 2010
Assets at fair value on a recurring basis
Assets other than temporary cash investments	$ 3,566.0	$ 3,596.0
Level 1 [Member] \| Equity securities [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	371.0	387.0
Level 1 [Member] \| Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	656.0	497.0
Level 1 [Member] \| Power contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	0	0
Level 1 [Member] \| Securitization recovery trust account [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	43.0	43.0
Level 1 [Member] \| Storm reserve escrow account [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	334.0	329.0
Level 1 [Member] \| Gas hedge contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Liabilities at fair value on a recurring basis
Liabilities, Fair Value Disclosure on Recurring Basis	16	2
Level 1 [Member] \| Power contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Liabilities at fair value on a recurring basis
Liabilities, Fair Value Disclosure on Recurring Basis	0	0
Level 1 [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Temporary cash investments	851	1,218
Total	2,255	2,474
Liabilities at fair value on a recurring basis
Total	16	2
Level 2 [Member] \| Equity securities [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	1,559.0	1,689.0
Level 2 [Member] \| Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	980.0	1,023.0
Level 2 [Member] \| Power contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	0	0
Level 2 [Member] \| Securitization recovery trust account [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	0	0
Level 2 [Member] \| Storm reserve escrow account [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	0	0
Level 2 [Member] \| Gas hedge contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Liabilities at fair value on a recurring basis
Liabilities, Fair Value Disclosure on Recurring Basis	0	0
Level 2 [Member] \| Power contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Liabilities at fair value on a recurring basis
Liabilities, Fair Value Disclosure on Recurring Basis	0	0
Level 2 [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Temporary cash investments	0	0
Total	2,539	2,712
Liabilities at fair value on a recurring basis
Total	0	0
Level 3 [Member] \| Equity securities [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	0	0
Level 3 [Member] \| Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	0	0
Level 3 [Member] \| Power contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	95.0	214.0
Level 3 [Member] \| Securitization recovery trust account [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	0	0
Level 3 [Member] \| Storm reserve escrow account [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	0	0
Level 3 [Member] \| Gas hedge contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Liabilities at fair value on a recurring basis
Liabilities, Fair Value Disclosure on Recurring Basis	0	0
Level 3 [Member] \| Power contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Liabilities at fair value on a recurring basis
Liabilities, Fair Value Disclosure on Recurring Basis	25	17
Level 3 [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Temporary cash investments	0	0
Total	95	214
Liabilities at fair value on a recurring basis
Total	25	17
Equity securities [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	1,930.0	2,076.0
Debt Securities [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	1,636.0	1,520.0
Power contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	95.0	214.0
Securitization recovery trust account [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	43.0	43.0
Storm reserve escrow account [Member] \| Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	334.0	329.0
Gas hedge contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Liabilities at fair value on a recurring basis
Liabilities, Fair Value Disclosure on Recurring Basis	16	2
Power contracts [Member] \| Fair Value, Measurements, Recurring [Member]
Liabilities at fair value on a recurring basis
Liabilities, Fair Value Disclosure on Recurring Basis	25	17
Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Temporary cash investments	851	1,218
Total	4,889	5,400
Liabilities at fair value on a recurring basis
Total	$ 41	$ 19

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15E

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15D

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 6: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15C

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 14

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 16

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

+ Details