Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Derivative [Line Items]
Derivative assets	$ 30	$ 15
Derivative liabilities	223	219
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	4	3
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	4	5
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	4	4
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	9
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	2	1
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	2	2
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	12
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	7
Power | Other Current Assets
Derivative [Line Items]
Derivative assets	13	4
Power | Other Assets
Derivative [Line Items]
Derivative assets	5	0
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	19	18
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	175	169
Union Electric Company
Derivative [Line Items]
Derivative assets	26	12
Derivative liabilities	15	22
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	4	3
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	4	5
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	4	4
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	5	9
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	4
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	1
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	13	4
Union Electric Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	5	0
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	3	4
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	4	3
Derivative liabilities	208	197
Ameren Illinois Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	0	0
Ameren Illinois Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets	2	1
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets	2	2
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	13	8
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	4	6
Ameren Illinois Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other Assets
Derivative [Line Items]
Derivative assets	0	0
Ameren Illinois Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities	16	14
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities	$ 175	$ 169