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	Jun. 30, 2012		Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Noncontrolling Interest	20.00%
Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Noncontrolling Interest	20.00%
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Preferred stock	$ 93	[1],[2]	$ 92	[1],[2]
Long-term debt (including current portion)	7,766	[1],[2]	7,800	[1],[2]
Fair Value [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Preferred stock	55		55
Long-term debt (including current portion)	4,619		4,541
Fair Value [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Preferred stock	38		37
Long-term debt (including current portion)	1,984		1,943
Fair Value [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	679		839
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	6,857	[1],[2]	6,856	[1],[2]
Preferred stock	142	[1],[2]	142	[1],[2]
Carrying Amount [Member] | Ameren Missouri [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	3,950		3,950
Preferred stock	80		80
Carrying Amount [Member] | Ameren Illinois Company [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	1,658		1,658
Preferred stock	62		62
Carrying Amount [Member] | Genco [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	$ 824		$ 824

[1] Includes amounts for Ameren registrant and nonregistrant subsidiaries and intercompany eliminations.

[2] Preferred stock not subject to mandatory redemption of the Ameren subsidiaries along with the 20% noncontrolling interest of EEI is recorded in Noncontrolling Interests on the balance sheet.