Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Details)	Dec. 31, 2013 gal		Dec. 31, 2012 gal
Fuel Oils
Derivative [Line Items]
Quantity	66,000,000	[1]	70,000,000	[1]
Natural Gas
Derivative [Line Items]
Quantity	136,000,000		147,000,000
Power
Derivative [Line Items]
Quantity	14,000,000		25,000,000
Uranium
Derivative [Line Items]
Quantity	796,000		446,000
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity	66,000,000	[1]	70,000,000	[1]
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity	28,000,000		19,000,000
Union Electric Company | Power
Derivative [Line Items]
Quantity	3,000,000		11,000,000
Union Electric Company | Uranium
Derivative [Line Items]
Quantity	796,000		446,000
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity	108,000,000		128,000,000
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity	11,000,000		14,000,000

[1] Fuel oils consist of heating oil, ultra-low-sulfur diesel, and crude oil.