Long-Term Debt And Equity Financings (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013
Long-Term Debt And Equity Financings [Line Items]
Excess in indebtedness upon default of maturity	$ 25
Ameren Missouri And Ameren Illinois [Member]
Long-Term Debt And Equity Financings [Line Items]
Assumed interest rate	6.00%
Dividend rate	7.00%
Ameren Illinois Company [Member] | Federal Energy Regulatory Commission Restriction [Member] | Actual Ratio [Member]
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to total capitalization	57.00%
Ameren Illinois Company [Member] | Minimum [Member] | Federal Energy Regulatory Commission Restriction [Member]
Long-Term Debt And Equity Financings [Line Items]
Common stock equity to total capitalization	30.00%