Short-Term Debt And Liquidity (Narrative) (Detail) (USD $)	3 Months Ended	6 Months Ended	3 Months Ended		9 Months Ended			6 Months Ended				6 Months Ended
	Jun. 30, 2012 Utilities [Member]	Jun. 30, 2012 Utilities [Member]	Jun. 30, 2012 Non State Regulated Subsidiaries [Member]	Jun. 30, 2011 Non State Regulated Subsidiaries [Member]	Jun. 30, 2012 Non State Regulated Subsidiaries [Member]	Jun. 30, 2011 Non State Regulated Subsidiaries [Member]	Jun. 30, 2012 Multiyear Credit Facility [Member] Maximum [Member]	Jun. 30, 2012 2010 Credit Agreements [Member]	Jun. 30, 2012 2010 Credit Agreements [Member] Ameren Missouri [Member]	Jun. 30, 2012 2010 Credit Agreements [Member] Ameren Illinois Company [Member]	Jun. 30, 2012 2010 Credit Agreements [Member] Genco [Member]	Jun. 30, 2012 Commercial Paper [Member]	Jun. 30, 2011 Commercial Paper [Member]
Line of Credit Facility [Line Items]
Aggregate amount of credit capacity available								$ 2,060,000,000
Commercial paper outstanding												30,000,000
Average daily commercial paper borrowings outstanding												72,000,000	338,000,000
Weighted average interest rate												0.94%	0.87%
Peak short-term borrowings												$ 229,000,000	$ 400,000,000
Peak short-term borrowings interest rate												1.25%	1.46%
Actual debt-to-capital ratio percentage							65	48	48	41	45
Minimum ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date								2.0
Current ratio of consolidated funds from operations plus interest expense to consolidated interest expense as of balance sheet date								5.1
Money pool, average interest rate	0.14%	0.12%	0.64%	0.72%	0.70%	0.93%