Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail)	Sep. 30, 2013 gal		Dec. 31, 2012 gal
Derivative That Qualify for Regulatory Deferral | Fuel Oils
Derivative [Line Items]
Accrual & NPNS Contracts	60,000,000	[1],[2]	70,000,000	[1],[2]
Derivative That Qualify for Regulatory Deferral | Natural Gas
Derivative [Line Items]
Accrual & NPNS Contracts	144,000,000	[1]	147,000,000	[1]
Derivative That Qualify for Regulatory Deferral | Power
Derivative [Line Items]
Accrual & NPNS Contracts	16,000,000	[1]	23,000,000	[1]
Derivative That Qualify for Regulatory Deferral | Uranium
Derivative [Line Items]
Accrual & NPNS Contracts	996,000	[1]	446,000	[1]
Derivative That Qualify for Regulatory Deferral | Union Electric Company | Natural Gas
Derivative [Line Items]
Accrual & NPNS Contracts	29,000,000	[1]	19,000,000	[1]
Derivative That Qualify for Regulatory Deferral | Union Electric Company | Power
Derivative [Line Items]
Accrual & NPNS Contracts	5,000,000	[1]	9,000,000	[1]
Derivative That Qualify for Regulatory Deferral | Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Accrual & NPNS Contracts	115,000,000	[1]	128,000,000	[1]
Derivative That Qualify for Regulatory Deferral | Ameren Illinois Company | Power
Derivative [Line Items]
Accrual & NPNS Contracts	11,000,000	[1]	14,000,000	[1]
Other Contract | Natural Gas
Derivative [Line Items]
Accrual & NPNS Contracts
Other Contract | Power
Derivative [Line Items]
Accrual & NPNS Contracts	1,000,000	[3]	2,000,000	[3]
Other Contract | Union Electric Company | Natural Gas
Derivative [Line Items]
Accrual & NPNS Contracts
Other Contract | Union Electric Company | Power
Derivative [Line Items]
Accrual & NPNS Contracts	1,000,000	[3]	2,000,000	[3]
Accrual and Normal Purchases and Normal Sales Contracts | Coal
Derivative [Line Items]
Accrual & NPNS Contracts	81,000,000	[4]	96,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts | Natural Gas
Derivative [Line Items]
Accrual & NPNS Contracts	8,000,000	[4]	20,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts | Power
Derivative [Line Items]
Accrual & NPNS Contracts	18,000,000	[4]	24,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts | 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 | Uranium
Derivative [Line Items]
Accrual & NPNS Contracts	4,516,000	[4]	5,142,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts | Union Electric Company | Natural Gas
Derivative [Line Items]
Accrual & NPNS Contracts	2,000,000	[4]	4,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts | Union Electric Company | Power
Derivative [Line Items]
Accrual & NPNS Contracts	3,000,000	[4]	3,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts | Union Electric Company | 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 | Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Accrual & NPNS Contracts	6,000,000	[4]	16,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts | Ameren Illinois Company | Power
Derivative [Line Items]
Accrual & NPNS Contracts	15,000,000	[4]	21,000,000	[4]
Accrual and Normal Purchases and Normal Sales Contracts | Ameren Illinois Company | Renewable Energy Credits [Member]
Derivative [Line Items]
Accrual & NPNS Contracts	11,000,000	[4],[5]	12,000,000	[4],[5]

[1] As of September 30, 2013, these contracts ran through October 2015, October 2019, May 2032, and October 2016 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 September 30, 2013, these contracts ran through December 2014 for power.

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