Long-Term Debt And Equity Financings (Narrative) (Detail) (USD $)	9 Months Ended		6 Months Ended	9 Months Ended						1 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Jun. 30, 2013 Union Electric Company [Member]	Sep. 30, 2012 Union Electric Company [Member]	Sep. 30, 2013 Ameren Missouri And Ameren Illinois [Member]	Sep. 30, 2013 Ameren Illinois Company	Sep. 30, 2012 Ameren Illinois Company	Sep. 30, 2013 Ameren Illinois Company Federal Energy Regulatory Commission Restriction [Member] Actual Ratio [Member]	Sep. 30, 2013 Ameren Illinois Company Minimum Federal Energy Regulatory Commission Restriction [Member]	Oct. 31, 2013 Subsequent Event [Member] Environmental Improvement And Pollution Control Revenue Bonds [Member] Series1993 Due2028 [Member] Union Electric Company [Member]	Oct. 31, 2013 Subsequent Event [Member] Secured Debt [Member] Senior Secured Notes465 Due2013 [Member] Union Electric Company [Member]
Long-Term Debt And Equity Financings [Line Items]
Repayments of Long-term Debt		$ 754,000,000	$ 0	$ 422,000,000			$ 332,000,000			$ 44,000,000	$ 200,000,000
Debt Instrument, Interest Rate, Stated Percentage										5.45%	4.65%
Assumed interest rate					6.00%
Dividend rate					7.00%
Excess in indebtedness upon default of maturity	$ 25,000,000
Common stock equity to total capitalization								58.00%	30.00%