Long-Term Debt and Equity Financings (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended		1 Months Ended	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011 Ameren Illinois [Member]	Jun. 30, 2011 Senior Unsecured Notes 8.875% Due 2014 [Member]
Common stock, shares issued	0.6	1.2
Common stock, value of shares issued	$ 15	$ 32	$ 43
Amount of senior notes matured and retired				$ 150
Interest rate on senior unsecured notes				6.625%
Excess in indebtedness upon default of maturity					25