Rate And Regulatory Matters Rate and Regulatory Matters (Narrative-Missouri) (Details)	1 Months Ended	3 Months Ended	12 Months Ended	36 Months Ended
	Apr. 30, 2015 USD ($)	Mar. 31, 2016	Dec. 31, 2015 USD ($) MWh mi	Feb. 28, 2019 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2012
Electric Distribution | Final Rate Order | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 122,000,000
Public Utilities, Approved Return on Equity, Percentage	9.53%
Public Utilities, Approved Equity Capital Structure, Percentage	51.80%
Rate Base	$ 7,000,000,000
Electric Distribution - Net Energy Costs [Member] | Final Rate Order | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Approved Rate Increase (Decrease), Amount	$ 109,000,000
Illinois Rivers Project [Member] | Ameren Transmission Company of Illinois [Member]
Rate And Regulatory Matters [Line Items]
Transmission Line Miles | mi			7
Mark Twain Project [Member] | Ameren Transmission Company of Illinois [Member]
Rate And Regulatory Matters [Line Items]
Transmission Line Miles | mi			100
MEEIA | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Energy Efficiency Program Spending					$ 134,000,000
Net Shared Benefits					174,000,000
Incentive Award if Energy Efficiency Goals Are Achieved					$ 19,000,000
Achieved Percentage of Energy Efficiency Earnings For Incentive Award			100.00%		100.00%	100.00%
Incentive Award if Energy Efficiency Goals Are Achieved, Period of Recognition					3 years
Subsequent Event | MEEIA | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Energy Efficiency Program Spending				$ 158,000,000
Incentive Award if Energy Efficiency Goals Are Achieved				$ 27,000,000
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		100.00%
Incentive Award if Energy Efficiency Goals Are Achieved, Period of Recognition				3 years
Incentive Award if Energy Efficiency Goals Are Achieved, Period				3 years
Subsequent Event | Minimum | MEEIA | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		25.00%
Noranda [Member] | Union Electric Company
Rate And Regulatory Matters [Line Items]
Revenue Requirement			$ 78,000,000		$ 78,000,000
Noranda Annual Megawatthours | MWh			4,200,000.0
Noranda Operating Capacity			100.00%
Noranda Summer Base Rate			$ 45.78
Noranda Winter Base Rate			$ 31.11
Noranda [Member] | Subsequent Event | Union Electric Company
Rate And Regulatory Matters [Line Items]
Customer pot lines Idled due to circuit failure		2
Number of customer pot lines		3
Number of days to file a rate case		60 days
Number of Months to complete MoPSC Electric Service Proceeding		11 months