Rate And Regulatory Matters (Schedule Of Regulatory Assets And Liabilities) (Details) (USD $) In Millions	Sep. 30, 2011		Dec. 31, 2010
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 1,213		$ 1,263
Regulatory liabilities	(1,464)		(1,319)
Ameren Missouri [Member] | Demand-Side Costs [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	33	[1],[2]
Ameren Missouri [Member] | Construction Accounting For Pollution Control Equipment [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	25	[1],[2]
Ameren Missouri [Member] | SO2 Emissions Allowances Sales Tracker [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	8	[2],[3]
Ameren Missouri [Member] | FERC-Ordered MISO Resettlements [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	2	[2],[3]
Ameren Missouri [Member] | 2006 Storm Costs [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	1	[2],[3]
Ameren Missouri [Member] | Vegetation Management And Infrastructure Inspection [Member]
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	(3)	[2],[3]
Ameren Missouri [Member] | Pension And Postretirement Benefit Cost Tracker For 2010 Costs [Member]
Rate And Regulatory Matters [Line Items]
Regulatory liabilities	(11)	[1],[2]
Ameren Missouri [Member]
Rate And Regulatory Matters [Line Items]
Regulatory assets	$ 55	[2]

[1] Recovery period first established in the MoPSC's July 2011 rate order.

[2] Represents amounts capitalized at implementation of the rate order at July 31, 2011, and excludes the impact of subsequent amortization of the regulatory assets or liabilities.

[3] Previous recovery period was extended.