Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Mar. 31, 2017		Dec. 31, 2016
Derivative [Line Items]
Derivative assets	[1]	$ 12		$ 27
Derivative liabilities	[2]	223		211
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		1		2
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		1		1
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		5		5
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		5		12
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets				3
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		8		4
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		15		10
Power | Other Current Assets
Derivative [Line Items]
Derivative assets		5		9
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		13		15
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		182		173
Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	[3]	4
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	7		14
Derivative liabilities	[2]	14		18
Union Electric Company | Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		1		2
Union Electric Company | Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets		1		1
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		5		5
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets				1
Union Electric Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets				1
Union Electric Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		2		1
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		6		5
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		5		9
Union Electric Company | Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		1		3
Union Electric Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0		0
Union Electric Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	[3]	4
Ameren Illinois Company
Derivative [Line Items]
Derivative assets	[1]	5		13
Derivative liabilities	[2]	209		193
Ameren Illinois Company | Fuel Oils | Other Current 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 | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		5		11
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets				2
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		5
Ameren Illinois Company | Natural Gas | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities		6		3
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		182		173
Ameren Illinois Company | Power | Mark To Market Derivative Liabilities
Derivative [Line Items]
Derivative liabilities		12		12
Ameren Illinois Company | Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		$ 0		$ 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.

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