Rate And Regulatory Matters (Narrative-Missouri) (Detail) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) mi	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Rate And Regulatory Matters [Line Items]
Electric		$ 1,725	$ 1,700	$ 4,101	$ 4,093
Union Electric Company
Rate And Regulatory Matters [Line Items]
Electric		1,144	$ 1,151	2,682	$ 2,752
Union Electric Company | MEEIA
Rate And Regulatory Matters [Line Items]
Electric		$ 19
Electric Distribution | Union Electric Company | MEEIA
Rate And Regulatory Matters [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved				$ 19
Achieved Percentage of Energy Efficiency Earnings For Incentive Award		100.00%		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
Noranda [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Regulatory Asset, Amortization Period		10 years
Revenue Requirement		$ 81
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
Pending Rate Case | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 206
Public Utilities, Requested Return on Equity, Percentage		9.90%
Public Utilities, Requested Equity Capital Structure, Percentage		51.80%
Rate Base		$ 7,200
Plant Additions to Rate Base		$ 1,400
Number of Months to complete MoPSC Electric Service Proceeding		11 months
Components of Rate Increase [Member] | Electric Distribution | Union Electric Company
Rate And Regulatory Matters [Line Items]
Public Utilities, Requested Rate Increase (Decrease), Amount		$ 74
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
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
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
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
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
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
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
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
Final Rate Order [Member] | Electric Distribution | Union Electric Company | MEEIA
Rate And Regulatory Matters [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved				$ 29
Subsequent Event | Final Rate Order [Member] | Electric Distribution | Union Electric Company | MEEIA
Rate And Regulatory Matters [Line Items]
Incentive Award if Energy Efficiency Goals Are Achieved	$ 28
Electric	$ 9