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, 2017 gal MWh MMBTU lb	Dec. 31, 2016 gal MWh MMBTU lb
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	30	30
Natural Gas
Derivative [Line Items]
Quantity | MMBTU		169	154
Power
Derivative [Line Items]
Quantity | MWh		11	10
Uranium
Derivative [Line Items]
Quantity | lb		370	345
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	30	30
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		24	25
Union Electric Company | Power
Derivative [Line Items]
Quantity | MWh		2	1
Union Electric Company | Uranium
Derivative [Line Items]
Quantity | lb		370	345
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity | MMBTU		145	129
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity | MWh		9	9

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