Commitments And Contingencies (Schedule Of Lease Obligations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Commitments and Contingencies [Line Items]
Capital lease payments, 2013	$ 32	[1],[2]
Capital lease payments, 2014	32	[1],[2]
Capital lease payments, 2015	33	[1],[2]
Capital lease payments, 2016	33	[1],[2]
Capital lease payments, 2017	33	[1],[2]
Capital lease payments, After 5 Years	425	[1],[2]
Capital lease payments, Total	588	[1],[2]
Less Amount representing interest, 2013	27	[2]
Less Amount representing interest, 2014	27	[2]
Less Amount representing interest, 2015	27	[2]
Less Amount representing interest, 2016	27	[2]
Less Amount representing interest, 2017	27	[2]
Less Amount representing interest, After 5 Years	149	[2]
Less Amount representing interest, Total	284	[2]
Present value of minimum capital lease payments, 2013	5	[2]
Present value of minimum capital lease payments, 2014	5	[2]
Present value of minimum capital lease payments, 2015	6	[2]
Present value of minimum capital lease payments, 2016	6	[2]
Present value of minimum capital lease payments, 2017	6	[2]
Present value of minimum capital lease payments, After 5 Years	276	[2]
Present value of minimum capital lease payments, Total	304	[2]
Operating leases, 2013	31	[2],[3]
Operating leases, 2014	27	[2],[3]
Operating leases, 2015	26	[2],[3]
Operating leases, 2016	26	[2],[3]
Operating leases, 2017	25	[2],[3]
Operating leases, After 5 Years	137	[2],[3]
Operating leases, Total	272	[2],[3]
Total lease obligations, 2013	36	[2]
Total lease obligations, 2014	32	[2]
Total lease obligations, 2015	32	[2]
Total lease obligations, 2016	32	[2]
Total lease obligations, 2017	31	[2]
Total lease obligations, After 5 Years	413	[2]
Total lease obligations, Total	576	[2]
Annual obligation for real estate leases and railroad licenses	2
Total rental expense	48	[4]	47	[4]	52	[4]
Union Electric Company [Member]
Commitments and Contingencies [Line Items]
Capital lease payments, 2013	32	[1]
Capital lease payments, 2014	32	[1]
Capital lease payments, 2015	33	[1]
Capital lease payments, 2016	33	[1]
Capital lease payments, 2017	33	[1]
Capital lease payments, After 5 Years	425	[1]
Capital lease payments, Total	588	[1]
Less Amount representing interest, 2013	27
Less Amount representing interest, 2014	27
Less Amount representing interest, 2015	27
Less Amount representing interest, 2016	27
Less Amount representing interest, 2017	27
Less Amount representing interest, After 5 Years	149
Less Amount representing interest, Total	284
Present value of minimum capital lease payments, 2013	5
Present value of minimum capital lease payments, 2014	5
Present value of minimum capital lease payments, 2015	6
Present value of minimum capital lease payments, 2016	6
Present value of minimum capital lease payments, 2017	6
Present value of minimum capital lease payments, After 5 Years	276
Present value of minimum capital lease payments, Total	304
Operating leases, 2013	12	[3]
Operating leases, 2014	12	[3]
Operating leases, 2015	12	[3]
Operating leases, 2016	12	[3]
Operating leases, 2017	13	[3]
Operating leases, After 5 Years	62	[3]
Operating leases, Total	123	[3]
Total lease obligations, 2013	17
Total lease obligations, 2014	17
Total lease obligations, 2015	18
Total lease obligations, 2016	18
Total lease obligations, 2017	19
Total lease obligations, After 5 Years	338
Total lease obligations, Total	427
Annual obligation for real estate leases and railroad licenses	1
Total rental expense	29		29		29
Ameren Illinois Company [Member]
Commitments and Contingencies [Line Items]
Operating leases, 2013	1	[3]
Operating leases, 2014	1	[3]
Operating leases, 2015	1	[3]
Operating leases, 2016	1	[3]
Operating leases, 2017	1	[3]
Operating leases, After 5 Years	2	[3]
Operating leases, Total	7	[3]
Annual obligation for real estate leases and railroad licenses	1
Total rental expense	$ 19		$ 17		$ 19

[1] See Properties under Part I, Item 2, and Note 3 - Property and Plant, Net of this report for additional information.

[2] ncludes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

[3] Amounts related to certain land-related leases have indefinite payment periods. The annual obligation of $2 million, $1 million and $1 million for Ameren, Ameren Missouri and Ameren Illinois for these items is included in the 2013 through 2017 columns, respectively.

[4] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.