Fair Value Measurements (Schedule Of Carrying Amounts And Estimated Fair Values Of Long-Term Debt And Preferred Stock) (Detail) - USD ($) $ in Millions		Sep. 30, 2017	Dec. 31, 2016
Fair Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	[1]	$ 8,234	$ 7,772
Preferred stock	[1]	131	131
Fair Value | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		4,312	4,304
Preferred stock		79	79
Fair Value | Ameren Illinois Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		2,759	2,765
Preferred stock		52	52
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)	[1]	7,699	7,276
Preferred stock	[1]	142	142
Carrying Amount | Union Electric Company
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term debt (including current portion)		3,967	3,994
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)		2,590	2,588
Preferred stock		$ 62	$ 62

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