Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Jun. 30, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative assets	[1]	$ 21	$ 18
Derivative liabilities	[2]	214	264
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		12	22
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		2	7
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		2	1
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		4	1
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		14	38
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		12	26
Power | Other Current Assets
Derivative [Line Items]
Derivative assets		15	16
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		13	13
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		157	157
Uranium | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		1	1
Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		3
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	16	17
Derivative liabilities	[2]	28	44
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		12	22
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		2	7
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1	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		15	16
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		1
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		1	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]	5	1
Derivative liabilities	[2]	186	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		3
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		6	18
Ameren Illinois Company | Natural Gas | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities		11	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		157	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] Because all contracts qualifying for hedge accounting receive regulatory deferral, the cumulative amount of pretax net gains on all derivative instruments is deferred as a regulatory liability.

[2] Because all contracts qualifying for hedge accounting receive regulatory deferral, the cumulative amount of pretax net losses on all derivative instruments is deferred as a regulatory asset.