Long-Term Debt And Equity Financings (Narrative) (Detail)	6 Months Ended
Jun. 30, 2016 USD ($)
Long-Term Debt And Equity Financings [Line Items]
Debt Default Provision Excess	$ 25,000,000
Ameren Missouri And Ameren Illinois
Long-Term Debt And Equity Financings [Line Items]
Assumed interest rate	5.00%
Dividend rate	6.00%
Secured Debt \| Senior Secured Notes540 Due2016 [Member] \| Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	5.40%
Repayments of Other Long-term Debt	$ 260,000,000
Secured Debt \| Senior Secured Notes, 3.65%, Due 2045 [Member] \| Union Electric Company
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Face Amount	$ 150,000,000
Debt Instrument, Interest Rate, Stated Percentage	3.65%
Proceeds from Issuance of Secured Debt	$ 148,000,000
Secured Debt \| Senior Secured Notes620 Due2016 [Member] \| Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.20%
Repayments of Other Long-term Debt	$ 54,000,000
Secured Debt \| Senior Secured Notes625 Due2016 [Member] \| Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	6.25%
Repayments of Other Long-term Debt	$ 75,000,000
Federal Energy Regulatory Commission Restriction [Member] \| Ameren Illinois Company
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to total capitalization	51.00%
Federal Energy Regulatory Commission Restriction [Member] \| Ameren Illinois Company \| Minimum
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to total capitalization	30.00%

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