Credit Facility Borrowings And Liquidity (Borrowing Activity On Credit Agreements) (Details) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2011
Line of Credit Facility [Line Items]
Peak credit facility borrowings during 2011	$ 440
2010 Missouri Credit Agreement [Member]
Line of Credit Facility [Line Items]
Average daily borrowings outstanding during 2011	140
Outstanding credit facility borrowings at period end
Weighted-average interest rate during 2011	2.30%
Peak credit facility borrowings during 2011	340	[1]
Peak interest rate during 2011	4.30%
2010 Genco Credit Agreement [Member]
Line of Credit Facility [Line Items]
Average daily borrowings outstanding during 2011	55
Outstanding credit facility borrowings at period end
Weighted-average interest rate during 2011	2.30%
Peak credit facility borrowings during 2011	$ 100	[1]
Peak interest rate during 2011	2.31%

[1] The timing of peak credit facility borrowings varies by company, and, therefore, the amounts presented by company might not equal the total peak credit facility borrowings for the period. The simultaneous peak credit facility borrowings by the Ameren Companies under all credit facilities during the first nine months of 2011 were $440 million.