Short-Term Debt And Liquidity (Narrative) (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Utilities
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.47%	0.08%
Credit Agreements 2012
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 1,500,000,000
Actual debt-to-capital ratio	0.52
Line of Credit Facility, Covenant Terms, Default Provision, Maximum Indebtedness	$ 75,000,000
Credit Agreements 2012 | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65
Interest Coverage Requirement	200.00%
Credit Agreements 2012 | Union Electric Company | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65
Credit Agreements 2012 | Ameren Illinois Company | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65
Missouri Credit Agreement 2012 | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.49
Illinois Credit Agreement 2012 | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.46