Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Sep. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative assets	[1]	$ 15	$ 18
Derivative liabilities	[2]	215	264
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		1
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		8	22
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		1	7
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		2	1
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1	1
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		12	38
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		15	26
Power | Other Current Assets
Derivative [Line Items]
Derivative assets		11	16
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		14	13
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		160	157
Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		2	1
Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		3
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	12	17
Derivative liabilities	[2]	25	44
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		1
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		8	22
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		1	7
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		3	6
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		6	8
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		11	16
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		2
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	0
Union Electric Company | Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		2	1
Union Electric Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		3
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	[1]	3	1
Derivative liabilities	[2]	190	220
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		1
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		0	0
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		9	18
Ameren Illinois Company | Natural Gas | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities		9	32
Ameren Illinois Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		0	0
Ameren Illinois Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		0	0
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		160	157
Ameren Illinois Company | Power | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities		12	13
Ameren Illinois Company | Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		0	$ 0
Ameren Illinois Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		$ 0

[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.