Debt and Credit Agreements (Details) (USD $)	Jun. 30, 2012	Dec. 31, 2011	Mar. 31, 2012 First and refunding mortgage bonds	Jun. 30, 2012 Revolving credit facility	Jun. 30, 2012 Letter of credit
Debt Instrument [Line Items]
Long-term debt - Gross			$ 400,000,000
Interest rate on debt (as a percent)			4.05%
Outstanding short-term debt	300,000,000	419,000,000
Short-term debt, weighted average interest rate (as a percent)	0.43%	0.44%
Credit facility				2,750,000,000
Credit facility, term				5 years
Outstanding credit facility					$ 63,000,000