Short-Term Debt And Liquidity (Narrative) (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.10%	0.10%	0.09%	0.23%
Credit Agreements 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,300,000,000		$ 1,300,000,000
Actual debt-to-capital ratio	0.5		0.5
Line of Credit Facility, Covenant Terms, Default Provision, Maximum Indebtedness	$ 75,000,000		$ 75,000,000
Credit Agreements 2012 | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65		0.65
Interest Coverage Requirement	200.00%		200.00%
Credit Agreements 2012 | Union Electric Company | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65		0.65
Credit Agreements 2012 | Ameren Illinois Company | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65		0.65
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.48		0.48
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.46		0.46