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, 2016 gal MWh MMBTU lb	Dec. 31, 2015 gal MWh MMBTU lb
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	25	35
Natural Gas
Derivative [Line Items]
Quantity | MMBTU		154	181
Power
Derivative [Line Items]
Quantity | MWh		10	11
Uranium
Derivative [Line Items]
Quantity | lb		445	494
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	25	35
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		26	30
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh		1	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb		445	494
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		128	151
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh		9	10

[1] Consists of ultra-low-sulfur diesel products.