Derivative Financial Instruments (Maximum Exposure If Counterparties Fail To Perform On Contracts) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 438		$ 790
Affiliates [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	71	[1]	276	[1]
Coal Producers [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	3		37
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	40		89
Electric Utilities [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	13		16
Financial Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	28		84
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	195		198
Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	3		3
Retail Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	85		87
Ameren Missouri [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	22		71
Ameren Missouri [Member] | Affiliates [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts		[1]	1	[1]
Ameren Missouri [Member] | Coal Producers [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts			35
Ameren Missouri [Member] | Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	2		1
Ameren Missouri [Member] | Electric Utilities [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	3		4
Ameren Missouri [Member] | Financial Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	14		26
Ameren Missouri [Member] | Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	3		4
Ameren Missouri [Member] | Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts
Ameren Missouri [Member] | Retail Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts
Ameren Illinois [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	1		85
Ameren Illinois [Member] | Affiliates [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts		[1]		[1]
Ameren Illinois [Member] | Coal Producers [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts
Ameren Illinois [Member] | Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts			84
Ameren Illinois [Member] | Electric Utilities [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts
Ameren Illinois [Member] | Financial Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	1		1
Ameren Illinois [Member] | Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts
Ameren Illinois [Member] | Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts
Ameren Illinois [Member] | Retail Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts
Other [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	415	[2]	634	[2]
Other [Member] | Affiliates [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	71	[1],[2]	275	[1],[2]
Other [Member] | Coal Producers [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	3	[2]	2	[2]
Other [Member] | Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	38	[2]	4	[2]
Other [Member] | Electric Utilities [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	10	[2]	12	[2]
Other [Member] | Financial Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	13	[2]	57	[2]
Other [Member] | Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	192	[2]	194	[2]
Other [Member] | Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	3	[2]	3	[2]
Other [Member] | Retail Companies [Member]
Concentration Risk [Line Items]
Maximum exposure to counterparties related to derivative contracts	$ 85	[2]	$ 87	[2]

[1] Primarily composed of Marketing Company’s exposure to Ameren Illinois related to financial contracts. The exposure is not eliminated at the consolidated Ameren level for purposes of this disclosure as it is calculated without regard to the offsetting affiliate counterparty’s liability position. See Note 14 - Related Party Transactions for additional information on these financial contracts.

[2] Includes amounts for Marketing Company, AERG, Genco, and AFS.