Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail)	Mar. 31, 2013		Dec. 31, 2012
Coal (In Tons) [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	91,000,000	[1]	96,000,000	[1]
Fuel Oils (In Gallons) [Member]
Derivative [Line Items]
Derivatives That Qualify for Regulatory Deferral	67,000,000	[2],[3]	26,000,000	[2],[3]
Natural Gas (In Mmbtu) [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	15,000,000	[1]	20,000,000	[1]
Other Derivatives	1,000,000	[4]
Derivatives That Qualify for Regulatory Deferral	158,000,000	[2]	147,000,000	[2]
Power (In Megawatt Hours) [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	23,000,000	[1]	24,000,000	[1]
Other Derivatives	2,000,000	[4]	2,000,000	[4]
Derivatives That Qualify for Regulatory Deferral	20,000,000	[2]	23,000,000	[2]
Renewable Energy Credits [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	15,000,000	[1],[5]	15,000,000	[1],[5]
Uranium (Pounds) [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	4,950,000	[1]	5,142,000	[1]
Derivatives That Qualify for Regulatory Deferral	480,000	[2]	446,000	[2]
Union Electric Company [Member] | Natural Gas (In Mmbtu) [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	3,000,000	[1]	4,000,000	[1]
Other Derivatives	1,000,000	[4]
Derivatives That Qualify for Regulatory Deferral	21,000,000	[2]	19,000,000	[2]
Union Electric Company [Member] | Power (In Megawatt Hours) [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	4,000,000	[1]	3,000,000	[1]
Other Derivatives	2,000,000	[4]	2,000,000	[4]
Derivatives That Qualify for Regulatory Deferral	8,000,000	[2]	9,000,000	[2]
Union Electric Company [Member] | Renewable Energy Credits [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	3,000,000	[1],[5]	3,000,000	[1],[5]
Ameren Illinois Company [Member] | Natural Gas (In Mmbtu) [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	12,000,000	[1]	16,000,000	[1]
Derivatives That Qualify for Regulatory Deferral	137,000,000	[2]	128,000,000	[2]
Ameren Illinois Company [Member] | Power (In Megawatt Hours) [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	19,000,000	[1]	21,000,000	[1]
Derivatives That Qualify for Regulatory Deferral	12,000,000	[2]	14,000,000	[2]
Ameren Illinois Company [Member] | Renewable Energy Credits [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	12,000,000	[1],[5]	12,000,000	[1],[5]

[1] Accrual contracts include commodity contracts that do not qualify as derivatives. As of March 31, 2013, these contracts ran through December 2017, March 2015, September 2024, May 2032, and October 2024 for coal, natural gas, power, renewable energy credits, and uranium, respectively.

[2] As of March 31, 2013, these contracts ran through October 2015, March 2017, May 2032, and September 2014 for fuel oils, natural gas, power, and uranium, respectively.

[3] Fuel oils consist of heating oil, ultra-low sulfur diesel, and crude oil.

[4] As of March 31, 2013, these contracts ran through April 2013 and December 2014 for natural gas and power, respectively.

[5] A renewable energy credit is created for every one megawatthour of renewable energy generated. The Ameren Companies’ contracts include renewable energy credits from solar and wind-generated power.