Commitments And Contingencies (Schedule Of Lease Obligations) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009
Commitments And Contingencies [Abstract]
Capital lease payments, 2012	$ 33	[1],[2]
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, After 5 Years	458	[1],[2]
Capital lease payments, Total	621	[1],[2]
Less Amount representing interest, 2012	28	[1]
Less Amount representing interest, 2013	27	[1]
Less Amount representing interest, 2014	27	[1]
Less Amount representing interest, 2015	27	[1]
Less Amount representing interest, 2016	27	[1]
Less Amount representing interest, After 5 Years	176	[1]
Less Amount representing interest, Total	312	[1]
Present value of minimum capital lease payments, 2012	5	[1]
Present value of minimum capital lease payments, 2013	5	[1]
Present value of minimum capital lease payments, 2014	5	[1]
Present value of minimum capital lease payments, 2015	6	[1]
Present value of minimum capital lease payments, 2016	6	[1]
Present value of minimum capital lease payments, After 5 Years	282	[1]
Present value of minimum capital lease payments, Total	309	[1]
Operating leases, 2012	38	[1],[3]
Operating leases, 2013	32	[1],[3]
Operating leases, 2014	26	[1],[3]
Operating leases, 2015	26	[1],[3]
Operating leases, 2016	25	[1],[3]
Operating leases, After 5 Years	160	[1],[3]
Operating leases, Total	307	[1],[3]
Total lease obligations, 2012	43	[1]
Total lease obligations, 2013	37	[1]
Total lease obligations, 2014	31	[1]
Total lease obligations, 2015	32	[1]
Total lease obligations, 2016	31	[1]
Total lease obligations, After 5 Years	442	[1]
Total lease obligations, Total	616	[1]
Annual obligation for real estate leases and railroad licenses	2
Total rental expense	$ 47	[1]	$ 52	[1]	$ 50	[1]

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

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

[3] Amounts related to certain real estate leases and railroad licenses have indefinite payment periods. Ameren's $2 million annual obligation for these items is included in the 2012 through 2016 columns. The amounts for the indefinite payments are not included in the After 5 Years column because that period is indefinite.