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Amount of decrease in unrecognized tax benefits resulting from tax positions that have been or will be taken in current period tax return.

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Amount of decrease in unrecognized tax benefits resulting from settlements with taxing authorities.

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Income Taxes (Narrative) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended			1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Jan. 31, 2011
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	$ 20	$ 0
State corporate income tax rate	4.00%	4.00%	5.00%
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	103
Maximum \| Illinois Corporate Income Tax
Income Taxes [Line Items]
State corporate income tax rate				9.50%
Union Electric Company
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	18	0
State corporate income tax rate	3.00%	3.00%	3.00%
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	95
Ameren Illinois Company
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities		0
State corporate income tax rate	6.00%	6.00%	6.00%
Unrecognized Tax Benefits, Decrease Resulting from Current Period Tax Positions	5
Income Tax Rate in 2015 \| Illinois Corporate Income Tax
Income Taxes [Line Items]
State corporate income tax rate	7.75%
Income Tax Rate in 2025 \| Illinois Corporate Income Tax
Income Taxes [Line Items]
State corporate income tax rate	7.30%
State
Income Taxes [Line Items]
Unrecognized Tax Benefits, Decrease Resulting from Settlements with Taxing Authorities	$ 2