Derivative Financial Instruments (Open Gross Derivative Volumes By Commodity Type) (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Fuel Oils
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 63	$ 66
Natural Gas
Derivative [Line Items]
Derivative Asset, Notional Amount	162	173
Power
Derivative [Line Items]
Derivative Asset, Notional Amount	12	9
Union Electric Company | Fuel Oils
Derivative [Line Items]
Derivative Asset, Notional Amount	63	66
Union Electric Company | Natural Gas
Derivative [Line Items]
Derivative Asset, Notional Amount	20	19
Union Electric Company | Power
Derivative [Line Items]
Derivative Asset, Notional Amount	4	1
Ameren Illinois Company | Fuel Oils
Derivative [Line Items]
Derivative Asset, Notional Amount	0	0
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Derivative Asset, Notional Amount	142	154
Ameren Illinois Company | Power
Derivative [Line Items]
Derivative Asset, Notional Amount	$ 8	$ 8