Derivative Financial Instruments (Potential Loss On Counterparty Exposures) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 398		$ 750
Affiliates [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	68	[1]	274	[1]
Coal Producers [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1		35
Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	30		88
Electric Utilities [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	5		9
Financial Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	21		65
Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	188		191
Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts			2
Retail Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	85		86
Ameren Missouri [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	15		66
Ameren Missouri [Member] | Affiliates [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts		[1]	1	[1]
Ameren Missouri [Member] | Coal Producers [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts			35
Ameren Missouri [Member] | Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1		1
Ameren Missouri [Member] | Electric Utilities [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1		3
Ameren Missouri [Member] | Financial Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	10		22
Ameren Missouri [Member] | Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	3		4
Ameren Missouri [Member] | Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts
Ameren Missouri [Member] | Retail Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts
Ameren Illinois [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts			84
Ameren Illinois [Member] | Affiliates [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts		[1]		[1]
Ameren Illinois [Member] | Coal Producers [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts
Ameren Illinois [Member] | Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts			84
Ameren Illinois [Member] | Electric Utilities [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts
Ameren Illinois [Member] | Financial Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts
Ameren Illinois [Member] | Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts
Ameren Illinois [Member] | Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts
Ameren Illinois [Member] | Retail Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts
Other [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	383	[2]	600	[2]
Other [Member] | Affiliates [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	68	[1],[2]	273	[1],[2]
Other [Member] | Coal Producers [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	1	[2]		[2]
Other [Member] | Commodity Marketing Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	29	[2]	3	[2]
Other [Member] | Electric Utilities [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	4	[2]	6	[2]
Other [Member] | Financial Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	11	[2]	43	[2]
Other [Member] | Municipalities/Cooperatives [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	185	[2]	187	[2]
Other [Member] | Oil And Gas Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts		[2]	2	[2]
Other [Member] | Retail Companies [Member]
Concentration Risk [Line Items]
Potential loss on counterparty exposures related to derivative contracts	$ 85	[2]	$ 86	[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.