Rate And Regulatory Matters (Narrative-Missouri) (Detail) - Subsequent Event Type [Domain] $ in Millions	1 Months Ended	3 Months Ended	6 Months Ended	12 Months Ended	30 Months Ended
	Apr. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2015 mi	Dec. 31, 2012 USD ($)	Jun. 30, 2015 USD ($)
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		$ 197		$ 147	$ 95
Net Shared Benefits		65		100	$ 115
Incentive Award if Energy Efficiency Goals Are Achieved		$ 30		$ 19
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		100.00%	100.00%	100.00%	100.00%
Incentive Award if Energy Efficiency Goals Are Achieved, Period of Recognition			2 years
Energy Efficiency Program Spending Period		3 years
Net Shared Benefits, Recognition Period		3 years
Incentive Award if Energy Efficiency Goals Are Achieved, Period		3 years
Minimum Percentage of Energy Efficiency Goal Achievement For Company To Be Eligible For Incentive Award		70.00%
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
Electric Distribution - Non-Energy Costs | Union Electric Company | Final Rate Order
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 13