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Related Party Transactions (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Related Party Transaction [Line Items]
Total Operating Revenues	$ 17	$ 12	$ 11
Ameren Missouri and Genco Gas Transportation Agreement [Member]
Related Party Transaction [Line Items]
Total Operating Revenues	1	1	1
Ameren Illinois Transmission Services Agreement with Marketing Company [Member]
Related Party Transaction [Line Items]
Total Operating Revenues	16	11	10
Ameren Illinois Power Supply Agreements with Marketing Company [Member]
Related Party Transaction [Line Items]
Operating Expenses	311	232	233
Ameren Illinois gas purchases from Genco [Member]
Related Party Transaction [Line Items]
Operating Expenses			$ 1

Amount of revenue, fees and commissions earned from transactions between (a) a parent company and its subsidiaries; (b) subsidiaries of a common parent; (c) an entity and trusts for the benefit of employees, for example, but not limited to, pension and profit-sharing trusts that are managed by or under the trusteeship of the entity's management; (d) an entity and its principal, owners, management, or members of their immediate families; and (e) affiliates.

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