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	74		73
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	50		49
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		4,006
Preferred stock	80		80
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		1,727
Preferred stock	$ 62		$ 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.