Derivative Financial Instruments (Derivative Instruments Carrying Value) (Detail) - Not Designated As Hedging Instrument - USD ($) $ in Millions		Jun. 30, 2017		Dec. 31, 2016
Derivative [Line Items]
Derivative assets	[1]	$ 18		$ 27
Derivative liabilities		221		211	[2]
Fuel Oils | Other Current Assets
Derivative [Line Items]
Derivative assets		1		2
Fuel Oils | Other Assets
Derivative [Line Items]
Derivative assets				1
Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		5		5
Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		1
Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		1		12
Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1		3
Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		11		4
Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		11		10
Power | Other Current Assets
Derivative [Line Items]
Derivative assets		14		9
Power | Other Assets
Derivative [Line Items]
Derivative assets		1
Power | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		14		15
Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		179		173
Uranium | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		0	[3]	4
Union Electric Company
Derivative [Line Items]
Derivative assets	[1]	16		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
Union Electric Company | Fuel Oils | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		5		5
Union Electric Company | Fuel Oils | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		1
Union Electric Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		0		1
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		2		1
Union Electric Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		5		5
Union Electric Company | Power | Other Current Assets
Derivative [Line Items]
Derivative assets		14		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		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]	2		13
Derivative liabilities	[2]	207		193
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
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
Ameren Illinois Company | Natural Gas | Other Current Assets
Derivative [Line Items]
Derivative assets		1		11
Ameren Illinois Company | Natural Gas | Other Assets
Derivative [Line Items]
Derivative assets		1		2
Ameren Illinois Company | Natural Gas | Other Current Liabilities
Derivative [Line Items]
Derivative liabilities		9		3
Ameren Illinois Company | Natural Gas | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		6		5
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		13		12
Ameren Illinois Company | Power | Other Deferred Credits And Liabilities
Derivative [Line Items]
Derivative liabilities		179		173
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

[3] Beginning in 2017, as a result of rulebook amendments at the Chicago Mercantile Exchange, the fair value of uranium derivative liabilities are offset by certain settlement payments made to the exchange previously characterized as collateral and included within “Other assets” on Ameren’s and Ameren Missouri’s balance sheet.