Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions		Sep. 30, 2015 gal MWh MMBTU lb	Dec. 31, 2014 gal MWh MMBTU lb
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	33	50
Natural Gas
Derivative [Line Items]
Quantity		182	136
Power
Derivative [Line Items]
Quantity | MWh		11	12
Uranium
Derivative [Line Items]
Quantity | lb		299	332
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	33	50
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity		30	28
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh		1	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb		299	332
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity		152	108
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh		10	11

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