Consolidated Balance Sheet (Parenthetical) (USD $) In Millions, except Per Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Accounts receivable - trade allowance for doubtful accounts	$ 22	$ 17
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	400.0	400.0
Common stock, shares outstanding	242.6	242.6
Union Electric Company [Member]
Accounts receivable - trade allowance for doubtful accounts	8	5
Common stock, par value	$ 5	$ 5
Common stock, shares authorized	150.0	150.0
Common stock, shares outstanding	102.1	102.1
Ameren Illinois Company [Member]
Accounts receivable - trade allowance for doubtful accounts	$ 14	$ 12
Common stock, no par value
Common stock, shares authorized	45.0	45.0
Common stock, shares outstanding	25.5	25.5