Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Detail) (USD $)
In Millions, unless otherwise specified

Mar. 31, 2013

Dec. 31, 2012

Fair Value [Member]

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Long-term debt (including current portion)

$ 7,127

^[1],[2]

$ 7,110

^[1],[2]

Preferred stock

124

^[1],[2]

123

^[1],[2]

Fair Value [Member] | Union Electric Company [Member]

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Long-term debt (including current portion)

4,645

4,625

Preferred stock

Fair Value [Member] | Ameren Illinois Company [Member]

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Long-term debt (including current portion)

2,027

2,020

Preferred stock

Carrying Amount [Member]

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Long-term debt (including current portion)

6,158

^[1],[2]

6,157

^[1],[2]

Preferred stock

142

^[1],[2]

142

^[1],[2]

Carrying Amount [Member] | Union Electric Company [Member]

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Long-term debt (including current portion)

4,006

Preferred stock

Carrying Amount [Member] | Ameren Illinois Company [Member]

Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]

Long-term debt (including current portion)

1,727

Preferred stock

$ 62

[1]	Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.
[2]	Preferred stock along with the noncontrolling interest of EEI is recorded in “Noncontrolling Interests” on the balance sheet.

Ameren Illinois Co 10-Q