Long-Term Debt And Equity Financings (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Jun. 30, 2012 Ameren Missouri And Ameren Illinois [Member]	Jun. 30, 2012 Ameren Illinois Company [Member]	Jun. 30, 2012 Ameren Illinois Company [Member] Federal Energy Regulatory Commission Restriction [Member] Actual [Member]	Jun. 30, 2012 Ameren Illinois Company [Member] Federal Energy Regulatory Commission Restriction [Member] Minimum [Member]	Jun. 30, 2012 Ameren Illinois Company [Member] 9.75% Senior Secured Notes Due 2018 [Member]	Jun. 30, 2012 Ameren Illinois Company [Member] 6.25% Senior Secured Notes Due 2018 [Member]
Long-Term Debt And Equity Financings [Line Items]
Debt instrument interest rate						9.75%	6.25%
Maturity year						2018	2018
Aggregate purchase price of tendered notes			$ 450
Assumed interest rate		6.00%
Dividend rate		7.00%
Excess in indebtedness upon default of maturity	$ 25
Common stock equity to total capitalization				58.00%	30.00%