Rate and Regulatory Matters (Schedule of Regulatory Assets and Liabilities) (Details) (USD $) In Millions	Jun. 30, 2011		Dec. 31, 2010
Regulatory assets	$ 1,224		$ 1,263
Regulatory liabilities	1,424		1,319
Ameren Missouri [Member] | Demand Side Costs [Member]
Regulatory assets	33	[1]
Ameren Missouri [Member] | Construction Accounting For Pollution Control Equipment [Member]
Regulatory assets	25	[1]
Ameren Missouri [Member] | SO2 Emissions Allowances Sales Tracker [Member]
Regulatory assets	8	[2]
Ameren Missouri [Member] | FERC-Ordered MISO Resettlements [Member]
Regulatory assets	2	[2]
Ameren Missouri [Member] | 2006 Storm Costs [Member]
Regulatory assets	1	[2]
Ameren Missouri [Member] | Vegetation Management And Infrastructure Inspection [Member]
Regulatory liabilities	3	[2]
Ameren Missouri [Member] | Pension And Postretirement Benefit Cost Tracker For 2010 Costs [Member]
Regulatory liabilities	11	[1]
Ameren Missouri [Member]
Regulatory assets	$ 55

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

[2] Previous recovery period was extended.