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Rate And Regulatory Matters (Narrative-Federal) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Ameren Illinois Company [Member]	Dec. 31, 2011 Ameren Illinois Company [Member]	Jan. 31, 2011 Wholesale Distribution Rate Case [Member] Ameren Illinois Company [Member]	Mar. 31, 2012 New Nuclear Energy Center [Member] Ameren Missouri [Member]
Utility revenue increase requested					$ 11
Capitalized costs relating to construction of new nuclear unit	$ 17,535	$ 18,127	$ 4,804	$ 4,770		$ 69

Tangible assets that are held by an entity for use in the production or supply of goods and services, for rental to others, or for administrative purposes and that are expected to provide economic benefit for more than one year; net of accumulated depreciation. Examples include land, buildings, machinery and equipment, and other types of furniture and equipment including, but not limited to, office equipment, furniture and fixtures, and computer equipment and software.

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