Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Derivative assets	[1]	$ 14	$ 17
Derivative liabilities	[2]	230	226
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		6	5
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		2	2
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		18	17
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		20	13
Power | Other Current Assets
Derivative [Line Items]
Derivative assets		4	9
Power | Other Assets
Derivative [Line Items]
Derivative assets		1
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		15	14
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		177	182
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	13	17
Derivative liabilities	[2]	10	9
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		6	5
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		2	2
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		0	1
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		5	5
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		4	3
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		4	9
Union Electric Company | Power | Other Assets
Derivative [Line Items]
Derivative assets		1
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		1	1
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	0
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	[1]	1	0
Derivative liabilities	[2]	220	217
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 | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1	0
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		13	12
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		16	10
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
Ameren Illinois Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		14	13
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		$ 177	$ 182

[1] The cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability.

[2] The cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.