Risk Management and Fair Values (Details 2) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Assets at fair value on a recurring basis
Assets other than temporary cash investments	$ 3,788.0	$ 3,596.0
Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Temporary cash investments	613	1,218
Total	5,098	5,400
Liabilities at fair value on a recurring basis
Total		19
Gas hedge contracts [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities at fair value on a recurring basis
Liabilities, Fair Value Disclosure on Recurring Basis	30	2
Power contracts [Member] | Fair Value, Measurements, Recurring [Member]
Liabilities at fair value on a recurring basis
Liabilities, Fair Value Disclosure on Recurring Basis		17
Equity securities [Member] | Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Assets other than temporary cash investments	2,129.0	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	1,659.0	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	312.0	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	50.0	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	335.0	329.0
Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Temporary cash investments	613	1,218
Total	2,034	2,474
Liabilities at fair value on a recurring basis
Total		2
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	30	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
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	397.0	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	639.0	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	50.0	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	335.0	329.0
Level 2 [Member] | Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Temporary cash investments	0	0
Total	2,752	2,712
Liabilities at fair value on a recurring basis
Total		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
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	1,732.0	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	1,020.0	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 3 [Member] | Fair Value, Measurements, Recurring [Member]
Assets at fair value on a recurring basis
Temporary cash investments	0	0
Total	312	214
Liabilities at fair value on a recurring basis
Total		17
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		17
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	312.0	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