Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail)	Mar. 31, 2014 gal		Dec. 31, 2013 gal
Fuel Oils
Derivative [Line Items]
Quantity	53,000,000	[1]	66,000,000	[1]
Natural Gas
Derivative [Line Items]
Quantity	137,000,000		136,000,000
Power
Derivative [Line Items]
Quantity	13,000,000		14,000,000
Uranium
Derivative [Line Items]
Quantity	827,000		796,000
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity	53,000,000	[1]	66,000,000	[1]
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity	28,000,000		28,000,000
Union Electric Company | Power
Derivative [Line Items]
Quantity	2,000,000		3,000,000
Union Electric Company | Uranium
Derivative [Line Items]
Quantity	827,000		796,000
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity	109,000,000		108,000,000
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity	11,000,000		11,000,000

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