Short-Term Debt And Liquidity (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.49
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	2.0
Current ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	5
Multiyear Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum consolidated indebtedness as a percent of total capitalization	65.00%
2010 Credit Agreements [Member] | Multiyear Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,960,000,000
Commercial Paper [Member]
Line of Credit Facility [Line Items]
Commercial paper outstanding	126,000,000		148,000,000
Average daily commercial paper borrowings outstanding	84,000,000	321,000,000
Weighted average interest rate	0.94%	0.94%
Peak short-term borrowings	$ 186,000,000	$ 377,000,000
Peak short-term borrowings interest rate	1.25%	1.46%