Derivative Financial Instruments (Narrative) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Cash collateral held from counterparties	$ 3	$ 1
Counterparty letters of credit held as collateral	7	9
Ameren Energy Marketing Company [Member]
Derivative [Line Items]
Cash collateral held from counterparties		1
Counterparty letters of credit held as collateral	6	7
Ameren Missouri [Member]
Derivative [Line Items]
Counterparty letters of credit held as collateral	1	1
Ameren Energy Generating Company [Member]
Derivative [Line Items]
Counterparty letters of credit held as collateral		1
Ameren Illinois [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities - affiliates		200
MTM derivative liabilities [Member] | Ameren Illinois [Member]
Derivative [Line Items]
Mark-to-market derivative liabilities - affiliates		$ 200