Rate And Regulatory Matters (Narrative-Missouri) (Detail)	3 Months Ended	6 Months Ended	12 Months Ended
	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($) mi	Dec. 31, 2015 USD ($) MWh
Electric Distribution | Union Electric Company | MEEIA
Rate And Regulatory Matters [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved		$ 19,000,000
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		100.00%
Incentive Award if Energy Efficiency Goals Are Achieved, Period of Recognition		3 years
Mark Twain Project | Ameren Transmission Company of Illinois
Rate And Regulatory Matters [Line Items]
Transmission Line Miles | mi		95
Noranda [Member] | Union Electric Company
Rate And Regulatory Matters [Line Items]
Revenue Requirement			$ 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
Subsequent Event | Noranda [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory Asset, Amortization Period	10 years
Revenue Requirement	$ 81,000,000
Subsequent Event | Components of Rate Increase - Return on Rate Base [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	25,000,000
Subsequent Event | Pending Rate Case | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 206,000,000
Public Utilities, Requested Return on Equity, Percentage	9.90%
Public Utilities, Requested Equity Capital Structure, Percentage	51.80%
Rate Base	$ 7,200,000,000
Plant Additions to Rate Base	$ 1,400,000,000
Number of Months to complete MoPSC Electric Service Proceeding	11 months
Subsequent Event | Components of Rate Increase [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 74,000,000
Subsequent Event | Components of Rate Increase - Depreciation Expense [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	39,000,000
Subsequent Event | Components of Rate Increase - Property Taxes [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	10,000,000
Subsequent Event | Components of Rate Increase - Customer Sales Volumes [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	51,000,000
Subsequent Event | Components of Rate Increase - Transmission Charges [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	34,000,000
Subsequent Event | Components of Rate Increase - Pension and Other Post-Employment Benefit Plan Expenses [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	24,000,000
Subsequent Event | Components of Rate Increase - Solar Rebate Expenses [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	15,000,000
Subsequent Event | Components of Rate Increase - Net Energy Costs [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	23,000,000
Subsequent Event | Components of Rate Increase - Income Taxes [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount	$ 15,000,000