Related Party Transactions (Narrative) (Details) $ in Millions	1 Months Ended			3 Months Ended	12 Months Ended
	Apr. 30, 2012 USD ($)	May. 31, 2011 MWh $ / MWh	Apr. 30, 2010 USD ($)	Dec. 31, 2015 MWh $ / MWh	Dec. 31, 2015 USD ($) MWh $ / MWh	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Union Electric Company
Related Party Transaction [Line Items]
Noncash Or Part Noncash Capital Contribution From Parent | $					$ 38	$ 9	$ 0
Period Five | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount | $	$ 1
Period Five | Ameren Illinois Company | Union Electric Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh		40,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh		28
Period Six | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount | $	$ 3
Period Seven [Member] | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount | $					$ 15
Period Four | Union Electric Company
Related Party Transaction [Line Items]
Energy Supply Agreements Amount | $			$ 1
Period Four | Ameren Illinois Company | Union Electric Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh		40,800
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh		29
Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh					168,400
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh					51
April 2015 Procurement [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh				667,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh				36
September 2015 Procurement [Member] | Ameren Illinois Power Supply Agreements with Ameren Missouri | Ameren Illinois Company
Related Party Transaction [Line Items]
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power | MWh				339,000
Long-term Contract for Purchase of Electric Power, Related Party Contract, Fixed Power, Rate | $ / MWh				38