Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	1 Months Ended	9 Months Ended	12 Months Ended	33 Months Ended
	Apr. 30, 2015 USD ($)	Sep. 30, 2015 mi	Dec. 31, 2012 USD ($)	Sep. 30, 2015 USD ($)
Union Electric Company
Rate And Regulatory Matters [Line Items]
Percentage of Customer's Sales Volume		5.00%
Electric Distribution | Union Electric Company | MEEIA
Rate And Regulatory Matters [Line Items]
Energy Efficiency Program Spending			$ 147	$ 110
Net Shared Benefits			100	$ 134
Incentive Award if Energy Efficiency Goals Are Achieved			$ 19
Achieved Percentage of Energy Efficiency Earnings For Incentive Award			100.00%
Incentive Award if Energy Efficiency Goals Are Achieved, Period of Recognition		3 years
Electric Distribution | Union Electric Company | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 122
Public Utilities, Approved Return on Equity, Percentage	9.53%
Public Utilities, Approved Equity Capital Structure, Percentage	51.80%
Rate Base	$ 7,000
Illinois Rivers Project | Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Transmission Line Miles | mi		7
Mark Twain Project | Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Transmission Line Miles | mi		100
Electric Distribution - Net Energy Costs | Union Electric Company | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 109