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, 2011		Dec. 31, 2010
Noncontrolling interest	20.00%
Carrying Amount [Member]
Long-term debt and capital lease obligations (including current portion)	$ 6,856	[1],[2]	$ 7,008	[1],[2]
Preferred stock	142	[1],[2]	142	[1],[2]
Fair Value [Member]
Long-term debt and capital lease obligations (including current portion)	7,800	[1],[2]	7,661	[1],[2]
Preferred stock	$ 92	[1],[2]	$ 102	[1],[2]

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

[2] Preferred stock along with the 20% noncontrolling interest of EEI is recorded in Noncontrolling Interests on the balance sheet.