Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail)	Jun. 30, 2012		Dec. 31, 2011
Uranium (Pounds In Thousands) [Member]
Derivative [Line Items]
NPNS Contract	5,361,000	[1]	5,553,000	[1]
Derivatives That Qualify for Regulatory Deferral	131,000	[2]	148,000	[2]
Power (In Megawatt Hours) [Member]
Derivative [Line Items]
NPNS Contract	95,000,000	[1]	73,000,000	[1]
Cash Flow Hedges	17,000,000	[3]	17,000,000	[3]
Other Derivatives	57,000,000	[4]	31,000,000	[4]
Derivatives That Qualify for Regulatory Deferral	28,000,000	[2]	21,000,000	[2]
Natural Gas (In Mmbtu) [Member]
Derivative [Line Items]
NPNS Contract	33,000,000	[1]	50,000,000	[1]
Other Derivatives	43,000,000	[4]	17,000,000	[4]
Derivatives That Qualify for Regulatory Deferral	175,000,000	[2]	193,000,000	[2]
Fuel Oils (In Gallons) [Member]
Derivative [Line Items]
Other Derivatives	55,000,000	[4],[5]	36,000,000	[4],[5]
Derivatives That Qualify for Regulatory Deferral	59,000,000	[2],[5]	53,000,000	[2],[5]
Coal (In Tons) [Member]
Derivative [Line Items]
NPNS Contract	146,000,000	[1]	147,000,000	[1]
Other Derivatives	6,000,000	[4]
Ameren Missouri [Member] | Uranium (Pounds In Thousands) [Member]
Derivative [Line Items]
NPNS Contract	5,361,000	[1]	5,553,000	[1]
Derivatives That Qualify for Regulatory Deferral	131,000	[2]	148,000	[2]
Ameren Missouri [Member] | Power (In Megawatt Hours) [Member]
Derivative [Line Items]
NPNS Contract	4,000,000	[1]	1,000,000	[1]
Other Derivatives	1,000,000	[4]	1,000,000	[4]
Derivatives That Qualify for Regulatory Deferral	13,000,000	[2]	6,000,000	[2]
Ameren Missouri [Member] | Natural Gas (In Mmbtu) [Member]
Derivative [Line Items]
NPNS Contract	6,000,000	[1]	8,000,000	[1]
Other Derivatives	16,000,000	[4]	9,000,000	[4]
Derivatives That Qualify for Regulatory Deferral	22,000,000	[2]	19,000,000	[2]
Ameren Missouri [Member] | Fuel Oils (In Gallons) [Member]
Derivative [Line Items]
Derivatives That Qualify for Regulatory Deferral	59,000,000	[2],[5]	53,000,000	[2],[5]
Ameren Missouri [Member] | Coal (In Tons) [Member]
Derivative [Line Items]
NPNS Contract	106,000,000	[1]	116,000,000	[1]
Ameren Illinois Company [Member] | Power (In Megawatt Hours) [Member]
Derivative [Line Items]
NPNS Contract	22,000,000	[1]	11,000,000	[1]
Derivatives That Qualify for Regulatory Deferral	19,000,000	[2]	24,000,000	[2]
Ameren Illinois Company [Member] | Natural Gas (In Mmbtu) [Member]
Derivative [Line Items]
NPNS Contract	27,000,000	[1]	42,000,000	[1]
Derivatives That Qualify for Regulatory Deferral	153,000,000	[2]	174,000,000	[2]
Genco [Member] | Natural Gas (In Mmbtu) [Member]
Derivative [Line Items]
Other Derivatives	26,000,000	[4]	7,000,000	[4]
Genco [Member] | Fuel Oils (In Gallons) [Member]
Derivative [Line Items]
Other Derivatives	43,000,000	[4],[5]	27,000,000	[4],[5]
Genco [Member] | Coal (In Tons) [Member]
Derivative [Line Items]
NPNS Contract	31,000,000	[1]	24,000,000	[1]
Other Derivatives	5,000,000	[4]

[1] Contracts through December 2017, March 2015, September 2035, and October 2024 for coal, natural gas, power, and uranium, respectively, as of June 30, 2012.

[2] Contracts through October 2014, October 2016, May 2032, and December 2013 for fuel oils, natural gas, power, and uranium, respectively, as of June 30, 2012.

[3] Contracts through December 2016 for power as of June 30, 2012.

[4] Contracts through December 2014, October 2016, April 2015, and December 2016 for coal, fuel oils, natural gas, and power, respectively, as of June 30, 2012.

[5] Fuel oils consist of heating and crude oil.