Short-Term Debt And Liquidity (Borrowing Activity On Credit Agreements) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Line of Credit Facility [Line Items]
Peak credit facility borrowings	$ 460	$ 925
2010 Missouri Credit Agreement [Member]
Line of Credit Facility [Line Items]
Average daily borrowings outstanding	105	195
Outstanding credit facility borrowings at period end		340
Weighted-average interest rate	2.30%	2.31%
Peak credit facility borrowings	340	380
Peak interest rate	4.30%	2.31%
2010 Genco Credit Agreement [Member]
Line of Credit Facility [Line Items]
Average daily borrowings outstanding	41	90
Outstanding credit facility borrowings at period end		100
Weighted-average interest rate	2.30%	2.31%
Peak credit facility borrowings	$ 100	$ 385
Peak interest rate	2.31%	2.31%