Short-Term Debt And Liquidity (Narrative) (Detail) (USD $)	9 Months Ended			3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2013 Commercial Paper [Member]	Sep. 30, 2012 Commercial Paper [Member]	Dec. 31, 2012 Commercial Paper [Member]	Sep. 30, 2013 Utilities [Member]	Sep. 30, 2012 Utilities [Member]	Sep. 30, 2013 Utilities [Member]	Sep. 30, 2012 Utilities [Member]	Sep. 30, 2013 Non State Regulated Subsidiaries [Member]	Sep. 30, 2012 Non State Regulated Subsidiaries [Member]	Sep. 30, 2013 Non State Regulated Subsidiaries [Member]	Sep. 30, 2012 Non State Regulated Subsidiaries [Member]	Sep. 30, 2013 Credit Agreements 2012 [Member]	Sep. 30, 2013 Credit Agreements 2012 [Member] Maximum	Sep. 30, 2013 Credit Agreements 2012 [Member] Union Electric Company	Sep. 30, 2013 Credit Agreements 2012 [Member] Ameren Illinois Company
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity												$ 2,090,000,000
Commercial paper outstanding
Average daily commercial paper borrowings outstanding	26,000,000	58,000,000
Weighted average interest rate	0.52%	0.93%
Peak short-term borrowings	$ 92,000,000	$ 229,000,000
Peak short-term borrowings interest rate	0.85%	1.25%
Actual debt-to-capital ratio percentage												0.51	0.65	0.48	0.42
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date												2.0 to 1
Current ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date												5.1 to 1.0
Short Term Debt, Weighted Average Interest Rate During Period				0.05%	0.14%	0.08%	0.13%	0.36%	0.52%	0.29%	0.64%
Line of Credit Facility, Covenant Terms, Ratio of Consolidated Operational Funds to Consolidated Interest Expense, Minimum												2.0
Line of Credit Facility, Covenant Compliance, Ratio of Consolidated Operational Funds to Consolidated Interest Expense												5.1