Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) gal in Millions, MWh in Millions		Mar. 31, 2018 gal MWh	Dec. 31, 2017 gal MWh
Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	27	28
Natural Gas
Derivative [Line Items]
Quantity		169	163
Power
Derivative [Line Items]
Quantity		11	12
Union Electric Company | Fuel Oils
Derivative [Line Items]
Quantity | gal	[1]	27	28
Union Electric Company | Natural Gas
Derivative [Line Items]
Quantity		25	24
Union Electric Company | Power
Derivative [Line Items]
Quantity		2	3
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Quantity		144	139
Ameren Illinois Company | Power
Derivative [Line Items]
Quantity		9	9

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