Short-Term Debt And Liquidity (Narrative) (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Line of Credit Facility [Line Items]
Commercial paper outstanding
Commercial Paper [Member]
Line of Credit Facility [Line Items]
Average daily commercial paper borrowings outstanding	2,000,000	84,000,000
Weighted average interest rate	0.80%	0.94%
Peak short-term borrowings	21,000,000	186,000,000
Peak short-term borrowings interest rate	0.85%	1.25%
Utilities [Member]
Line of Credit Facility [Line Items]
Weighted average interest rate	0.11%	0.11%
Non State Regulated Subsidiaries [Member]
Line of Credit Facility [Line Items]
Weighted average interest rate	0.22%	0.76%
Credit Agreements 2012 [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 2,060,000,000
Actual debt-to-capital ratio percentage	0.52
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	2.0 to 1
Covenant terms, ratio of consolidated operational funds to consolidated interest expense, minimum
Current ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date	5.1 to 1.0
Covenant compliance, ratio of consolidated operational funds to consolidated interest expense
Credit Agreements 2012 [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio percentage	0.65
Credit Agreements 2012 [Member] | Union Electric Company [Member]
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio percentage	0.48
Credit Agreements 2012 [Member] | Ameren Illinois Company [Member]
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio percentage	0.42