Consolidated Balance Sheet (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Accounts receivable - trade, allowance for doubtful accounts	$ 20	$ 23
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	400,000,000	400,000,000
Common stock, shares outstanding	242,600,000	240,400,000
Union Electric Company [Member]
Accounts receivable - trade, allowance for doubtful accounts	7	8
Common stock, par value	$ 5	$ 5
Common stock, shares authorized	150,000,000	150,000,000
Common stock, shares outstanding	102,100,000	102,100,000
Ameren Illinois Company [Member]
Accounts receivable - trade, allowance for doubtful accounts	$ 13	$ 13
Common stock, no par value
Common stock, shares authorized	45,000,000	45,000,000
Common stock, shares outstanding	25,500,000	25,500,000
Ameren Energy Generating Company [Member]
Common stock, no par value
Common stock, shares authorized	10,000	10,000
Common stock, shares outstanding	2,000	2,000