Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) lb in Thousands, gal in Millions, MWh in Millions, MMBTU in Millions		Mar. 31, 2016 MWh gal MMBTU lb	Dec. 31, 2015 MWh gal MMBTU lb
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	29	35
Natural Gas
Derivative [Line Items]
Quantity | MMBTU		179	181
Power
Derivative [Line Items]
Quantity | MWh		10	11
Uranium
Derivative [Line Items]
Quantity | lb		428	494
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	29	35
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		29	30
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh		1	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb		428	494
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		150	151
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh		9	10

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