Selected Quarterly Information (Summary Of Selected Quarterly Information) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended						3 Months Ended								12 Months Ended			3 Months Ended								12 Months Ended			3 Months Ended	6 Months Ended	9 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2012 Ameren Illinois Company [Member]	Sep. 30, 2012 Ameren Illinois Company [Member]	Jun. 30, 2012 Ameren Illinois Company [Member]	Mar. 31, 2012 Ameren Illinois Company [Member]	Dec. 31, 2011 Ameren Illinois Company [Member]	Sep. 30, 2011 Ameren Illinois Company [Member]	Jun. 30, 2011 Ameren Illinois Company [Member]	Mar. 31, 2011 Ameren Illinois Company [Member]	Dec. 31, 2012 Ameren Illinois Company [Member]	Dec. 31, 2011 Ameren Illinois Company [Member]	Dec. 31, 2010 Ameren Illinois Company [Member]	Dec. 31, 2012 Union Electric Company [Member]	Sep. 30, 2012 Union Electric Company [Member]	Jun. 30, 2012 Union Electric Company [Member]	Mar. 31, 2012 Union Electric Company [Member]	Dec. 31, 2011 Union Electric Company [Member]	Sep. 30, 2011 Union Electric Company [Member]	Jun. 30, 2011 Union Electric Company [Member]	Mar. 31, 2011 Union Electric Company [Member]	Dec. 31, 2012 Union Electric Company [Member]	Dec. 31, 2011 Union Electric Company [Member]	Dec. 31, 2010 Union Electric Company [Member]	Mar. 31, 2012 Classification of Activity from Nuclear Decommissioning Trust Fund [Member]	Jun. 30, 2012 Classification of Activity from Nuclear Decommissioning Trust Fund [Member]	Sep. 30, 2012 Classification of Activity from Nuclear Decommissioning Trust Fund [Member]
Selected Quarterly Financial Information [Line Items]
Operating revenues	$ 1,509	[1]	$ 2,001	[1]	$ 1,660	[1]	$ 1,658	[1]	$ 1,578	[1]	$ 2,268	[1]	$ 1,781	[1]	$ 1,904	[1]	$ 6,828		$ 7,531		$ 7,638		$ 589	$ 648	$ 564	$ 724	$ 611	$ 745	$ 623	$ 808	$ 2,525	$ 2,787	$ 3,014	$ 673	$ 1,064	$ 844	$ 691	$ 674	$ 1,115	$ 822	$ 772	$ 3,272	$ 3,383	$ 3,197
Operating Income	(1,816)	[1],[2]	635	[1],[2]	363	[1],[2]	(422)	[1],[2]	148	[1],[2]	550	[1],[2]	316	[1],[2]	227	[1],[2]	(1,240)		1,241		916		51	151	86	89	75	196	99	88	377	458	498	69	429	269	78	23	333	176	77	845	609	711
Net income (loss)	(1,156)	[1]	374	[1]	211	[1]	(403)	[1]	25	[1]	285	[1]	138	[1]	71	[1]	(974)	[3]	519	[3]	139	[3]	12	71	33	28	26	98	38	34	144	196	252	16	237	144	22	(14)	191	91	22	419	290	369
Earnings per Common Share - Basic and Diluted	$ (4.76)	[1]	$ 1.54	[1]	$ 0.87	[1]	$ (1.66)	[1]	$ 0.10	[1]	$ 1.18	[1]	$ 0.57	[1]	$ 0.29	[1]	$ (4.01)		$ 2.15		$ 0.58
Net Income Available to Common Stockholder																							11	71	32	27	25	98	37	33	141	193	248	16	236	143	21	(14)	190	90	21	416	287	364
Impairment and other charges																	2,578	[4]	125	[4]	589	[4]
Quantifying Misstatement in Current Year Financial Statements, Amount																																													$ 14	$ 26	$ 49

[1] The sum of quarterly amounts, including per share amounts, may not equal amounts reported for year-to-date periods. This is due to the effects of rounding and changes in the number of weighted-average shares outstanding each period.

[2] Includes pretax "Impairment and other charges" of $2,578 million and $125 million recorded at Ameren during the years ended December 31, 2012, and 2011, respectively. See Note 17 - Impairment and Other Charges under Part II, Item 8, for additional information.

[3] Represents net income (loss) available to common stockholders.

[4] Includes amounts for registrant and nonregistrant subsidiaries.