Short-Term Debt And Liquidity (Narrative) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Utilities [Member]
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.08%	0.39%
Credit Agreements 2012 [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	1,100,000,000
Actual debt-to-capital ratio	0.51
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	2.0 to 1.0
Line of Credit Facility, Covenant Terms, Default Provision, Maximum Indebtedness	75,000,000
Credit Agreements 2012 [Member] | Maximum
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.65
Missouri Credit Agreement 2012 [Member] | Union Electric Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.5
Illinois Credit Agreement 2012 [Member] | Ameren Illinois Company
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio	0.46