Short-Term Debt And Liquidity (Narrative) (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Line of Credit Facility [Line Items]
Commercial paper outstanding
Commercial Paper [Member]
Line of Credit Facility [Line Items]
Commercial paper outstanding	25,000,000		25,000,000
Average daily commercial paper borrowings outstanding			13,000,000	72,000,000
Weighted average interest rate	0.54%	0.94%	0.54%	0.94%
Peak short-term borrowings			78,000,000	229,000,000
Peak short-term borrowings interest rate			0.85%	1.25%
Utilities [Member]
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.07%	0.14%	0.09%	0.12%
Non State Regulated Subsidiaries [Member]
Line of Credit Facility [Line Items]
Short Term Debt, Weighted Average Interest Rate During Period	0.29%	0.64%	0.26%	0.70%
Credit Agreements 2012 [Member]
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 2,060,000,000		$ 2,060,000,000
Actual debt-to-capital ratio percentage	0.52		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			2.0
Current ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date			4.9 to 1.0
Covenant compliance, ratio of consolidated operational funds to consolidated interest expense			4.9
Credit Agreements 2012 [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio percentage	0.65		0.65
Credit Agreements 2012 [Member] | Ameren Missouri [Member]
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio percentage	0.48		0.48
Credit Agreements 2012 [Member] | Ameren Illinois Company [Member]
Line of Credit Facility [Line Items]
Actual debt-to-capital ratio percentage	0.42		0.42