Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended				12 Months Ended	1 Months Ended		12 Months Ended				12 Months Ended				12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2012	Dec. 31, 2011 Illinois Corporate Income Tax	Jan. 31, 2011 Minimum Illinois Corporate Income Tax	Jan. 31, 2011 Maximum Illinois Corporate Income Tax	Dec. 31, 2013 Union Electric Company	Dec. 31, 2012 Union Electric Company	Dec. 31, 2011 Union Electric Company	Jun. 30, 2011 Union Electric Company	Dec. 31, 2013 Ameren Illinois Company	Dec. 31, 2012 Ameren Illinois Company	Dec. 31, 2011 Ameren Illinois Company	Jun. 30, 2011 Ameren Illinois Company	Dec. 31, 2011 Ameren Illinois Company Illinois Corporate Income Tax	Dec. 31, 2015 Income Tax Rate in 2015 Illinois Corporate Income Tax	Dec. 31, 2025 Income Tax Rate in 2025 Illinois Corporate Income Tax
Income Taxes [Line Items]
State corporate income tax rate	4.00%	5.00%	4.00%			7.30%	9.50%	3.00%	3.00%	3.00%		6.00%	6.00%	5.00%			7.75%	7.30%
Increase in current state and local tax expense benefit					$ 6											$ 4
Decrease in deferred state and local income tax expense benefit					2											3
Reduction of uncertain tax positions				39							17				12
Estimated Unrecognized Tax Benefits, Decreases Resulting From Settlements with Taxing Authorities	20							13
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	$ 103							$ 95				$ 5