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, 2014		Dec. 31, 2013
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	$ 6,648	[1]	$ 6,584	[1]
Preferred stock	120	[1]	118	[1]
Fair Value | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	4,464		4,124
Preferred stock	72		71
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	2,184		2,028
Preferred stock	48		47
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	5,944	[1]	6,038	[1]
Preferred stock	142	[1]	142	[1]
Carrying Amount | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	4,004		3,757
Preferred stock	80		80
Carrying Amount | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	1,940		1,856
Preferred stock	$ 62		$ 62

[1] Preferred stock is recorded in “Noncontrolling Interests” on the consolidated balance sheet.