Related Party Transactions (Narrative) (Detail) (USD $) In Millions, unless otherwise specified					3 Months Ended			0 Months Ended
	Mar. 31, 2013 people	Mar. 31, 2013 Medina Valley Energy Center [Member]	Mar. 31, 2013 Futures Commission Merchants [Member]	Mar. 31, 2013 Ameren Energy Marketing Company [Member]	Mar. 31, 2013 Ameren Energy Marketing Company [Member] Ameren Illinois Company [Member]	Mar. 31, 2013 Guarantee Type, Other [Member]	Apr. 30, 2013 Subsequent Event [Member] Futures Commission Merchants [Member]	Mar. 14, 2013 Elgin, Gibson City and Grand Tower Energy Centers [Member] Ameren Energy Generating Company [Member]	Mar. 14, 2013 New Ameren Energy Resources Company, LLC [Member]
Related Party Transaction [Line Items]
Proceeds from sale of property, plant, and equipment								$ 100
Accounts payable, related parties					9
Purchase of receivables from Marketing Company					33
Related Party Transaction, Purchases of Financing Receivable, Discount					1
Related Party Transaction, Guarantees Outstanding	269	33	65	158		13	25
Related Party Transaction, Guarantees, Number of Clearing Brokers	2
Related Party Transaction, Guarantees, Maximum Exposure				26
Letters of Credit Outstanding, Amount	37
Obligation to provide credit support, period									24 months
Buyer's indemnification guarantee obligation									$ 25
Buyer's indemnification guarantee obligation, period									24 months