Credit Facility Borrowings and Liquidity (Borrowing Activity on Credit Agreements) (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2011
Peak credit facility borrowings during 2011	$ 440
2010 Missouri Credit Agreement [Member]
Average daily borrowings outstanding during 2011	181
Outstanding credit facility borrowings at period end	200
Weighted-average interest rate during 2011	2.31%
Peak credit facility borrowings during 2011	340	[1]
Peak interest rate during 2011	4.30%
Genco Credit Agreement 2010 [Member]
Average daily borrowings outstanding during 2011	83
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 six months of 2011 were $440 million.