Short Term Credit Arrangements and Borrowings (Details Textuals) (USD $)	9 Months Ended	60 Months Ended
	Sep. 30, 2011 percent people years kWh Days weeks MCF banks customers ratio employees	Oct. 01, 2016 years banks customers ratio employees	Dec. 31, 2010
Short-term Debt [Line Items]
Revolving Credit Facilities with a Syndicate Number of Banks	23
Maximum Percentage of Commitment to Bank in any Facility	8.25%
Maximum Total Funded Debt to Total Capitalization Ratio	65.00%
Maximum Total Funded Debt to Total Capitilization Ration Denominator	100.00%
Letters of Credit Outstanding, Amount	$ 38,000,000
Line of Credit Facility, Current Borrowing Capacity	1,975,000,000
Renewal of Credit Facilities	1,800,000,000
Renewal of Credit Facility Time Period		5
Borrowing Capacity Under Financing Agreement With iss Clearing Agent	100,000,000
Maximum Additional Margin Financing	50,000,000
Line of Credit Facility, Increase, Additional Borrowings	15,000,000
Line of Credit Facility Current Borrowing	115,000,000
Other Short-term Borrowings	4,000,000		39,000,000
Parent [Member] | Unsecured Revolving Facility 1 [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity	538,000,000
Subsidiary 1 [Member] | Unsecured Revolving Facility 1 [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity	212,000,000
Subsidiary 2 [Member] | Unsecured Revolving Facility 1 [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity	250,000,000
Unsecured Revolving Facility 1 [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity	1,000,000,000
Parent [Member] | Unsecured Revolving Facility 2 [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity	562,000,000
Subsidiary 1 [Member] | Unsecured Revolving Facility 2 [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity	63,000,000
Subsidiary 2 [Member] | Unsecured Revolving Facility 2 [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity	175,000,000
Unsecured Revolving Facility 2 [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity	800,000,000
Parent [Member]
Short-term Debt [Line Items]
Total Funded Debt to Total Capitalization Ratio	49.00%
Total Funded Debt to Total Capitalization Ratio Denomenatory	100.00%
Line of Credit Facility, Current Borrowing Capacity	1,275,000,000
Subsidiary 1 [Member]
Short-term Debt [Line Items]
Total Funded Debt to Total Capitalization Ratio	52.00%
Total Funded Debt to Total Capitalization Ratio Denomenatory	100.00%
Line of Credit Facility, Current Borrowing Capacity	275,000,000
Subsidiary 2 [Member]
Short-term Debt [Line Items]
Total Funded Debt to Total Capitalization Ratio	47.00%
Total Funded Debt to Total Capitalization Ratio Denomenatory	100.00%
Line of Credit Facility, Current Borrowing Capacity	$ 425,000,000