Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail)	Jun. 30, 2013 MMBTU		Dec. 31, 2012 MMBTU
Derivativea That Qualify for Regulatory Deferral [Member] | Natural Gas [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	157,000,000	[1]	147,000,000	[1]
Derivativea That Qualify for Regulatory Deferral [Member] | Power [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	18,000,000	[1]	23,000,000	[1]
Derivativea That Qualify for Regulatory Deferral [Member] | Uranium [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	514,000	[1]	446,000	[1]
Derivativea That Qualify for Regulatory Deferral [Member] | Ameren Missouri [Member] | Fuel Oils [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	58,000,000	[1],[2]	70,000,000	[1],[2]
Derivativea That Qualify for Regulatory Deferral [Member] | Ameren Missouri [Member] | Natural Gas [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	30,000,000	[1]	19,000,000	[1]
Derivativea That Qualify for Regulatory Deferral [Member] | Ameren Missouri [Member] | Power [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	7,000,000	[1]	9,000,000	[1]
Derivativea That Qualify for Regulatory Deferral [Member] | Ameren Illinois Company [Member] | Natural Gas [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	127,000,000	[1]	128,000,000	[1]
Derivativea That Qualify for Regulatory Deferral [Member] | Ameren Illinois Company [Member] | Power [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	11,000,000	[1]	14,000,000	[1]
Other Contract [Member] | Natural Gas [Member]
Derivative [Line Items]
Accrual & NPNS Contracts
Other Contract [Member] | Power [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	1,000,000	[3]	2,000,000	[3]
Other Contract [Member] | Ameren Missouri [Member] | Natural Gas [Member]
Derivative [Line Items]
Accrual & NPNS Contracts
Other Contract [Member] | Ameren Missouri [Member] | Power [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	1,000,000	[3]	2,000,000	[3]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Coal [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	85,000,000	[4]	96,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Natural Gas [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	9,000,000	[4]	20,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Power [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	21,000,000	[4]	24,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Renewable Energy Credits [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	14,000,000	[4],[5]	15,000,000	[4],[5]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Uranium [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	4,671,000	[4]	5,142,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Ameren Missouri [Member] | Natural Gas [Member]
Derivative [Line Items]
Accrual & NPNS Contracts			4,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Ameren Missouri [Member] | Power [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	3,000,000	[4]	3,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Ameren Missouri [Member] | Renewable Energy Credits [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	3,000,000	[4],[5]	3,000,000	[4],[5]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Ameren Illinois Company [Member] | Natural Gas [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	9,000,000	[4]	16,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Ameren Illinois Company [Member] | Power [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	18,000,000	[4]	21,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts [Member] | Ameren Illinois Company [Member] | Renewable Energy Credits [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	11,000,000	[4],[5]	12,000,000	[4],[5]

[1] As of June 30, 2013, these contracts ran through October 2015, October 2019, May 2032, and May 2015 for fuel oils, natural gas, power, and uranium, respectively.

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

[3] As of June 30, 2013, these contracts ran through December 2014 for power.

[4] Accrual contracts include commodity contracts that do not qualify as derivatives. As of June 30, 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.

[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.