Fair Value Measurements (Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt and Capital Lease Obligations and Preferred Stock) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2011		Dec. 31, 2010
Noncontrolling interest	20.00%
Carrying Amount [Member]
Long-term debt and capital lease obligations (including current portion)	$ 6,859	[1],[2]	$ 7,008	[1],[2]
Preferred stock	142	[1],[2]	142	[1],[2]
Estimated of Fair Value [Member]
Long-term debt and capital lease obligations (including current portion)	7,666	[1],[2]	7,661	[1],[2]
Preferred stock	$ 102	[1],[2]	$ 102	[1],[2]

[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.