Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail)	Mar. 31, 2015 gal		Dec. 31, 2014 gal
Fuel Oils
Derivative [Line Items]
Quantity	40,000,000	[1]	50,000,000	[1]
Natural Gas
Derivative [Line Items]
Quantity	150,000,000		136,000,000
Power
Derivative [Line Items]
Quantity	11,000,000		12,000,000
Uranium
Derivative [Line Items]
Quantity	349,000		332,000
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity	40,000,000	[1]	50,000,000	[1]
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity	29,000,000		28,000,000
Union Electric Company | Power
Derivative [Line Items]
Quantity	1,000,000		1,000,000
Union Electric Company | Uranium
Derivative [Line Items]
Quantity	349,000		332,000
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity	121,000,000		108,000,000
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity	10,000,000		11,000,000

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