Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations	$ 6,981	[1],[2]	$ 6,856	[1],[2]
Preferred stock	142	[1],[2]	142	[1],[2]
Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	7,728	[1],[2]	7,800	[1],[2]
Preferred stock	123	[1],[2]	92	[1],[2]
Union Electric Company [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations	4,006		3,950
Preferred stock	80		80
Union Electric Company [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	4,625		4,541
Preferred stock	73		55
Ameren Illinois Company [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations	1,727		1,658
Preferred stock	62		62
Ameren Illinois Company [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	2,020		1,943
Preferred stock	49		37
Ameren Energy Generating Company [Member] | Carrying Amount [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt and Capital Lease Obligations	824		824
Ameren Energy Generating Company [Member] | Fair Value [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt and capital lease obligations (including current portion)	$ 618		$ 839

[1] Preferred stock along with the noncontrolling interest of EEI is recorded in "Noncontrolling Interests" on the balance sheet.

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