Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Detail) - USD ($) $ in Millions	Sep. 30, 2015		Dec. 31, 2014
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 89		$ 106
Current losses deferred as regulatory assets	163		295
Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	0
Current losses deferred as regulatory assets	19
Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(25)	[1]	(29)
Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	31
Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(57)	[2]	(54)
Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	22
Current losses deferred as regulatory assets	13
Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(148)	[3]	(130)
Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	1
Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(1)	[4]	(2)
Union Electric Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	41		18
Current losses deferred as regulatory assets	97		163
Union Electric Company | Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	0
Current losses deferred as regulatory assets	19
Union Electric Company | Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(25)	[1]	(29)
Union Electric Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	5
Union Electric Company | Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(13)	[2]	(11)
Union Electric Company | Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	22
Current losses deferred as regulatory assets	1
Union Electric Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	22	[3]	12
Union Electric Company | Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	1
Union Electric Company | Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(1)	[4]	(2)
Ameren Illinois Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	37		84
Current losses deferred as regulatory assets	65		129
Ameren Illinois Company | Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	0	[1]	0
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Current losses deferred as regulatory assets	26
Ameren Illinois Company | Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(44)	[2]	(43)
Ameren Illinois Company | Power
Derivative [Line Items]
Current losses deferred as regulatory assets	12
Ameren Illinois Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(170)	[3]	(142)
Ameren Illinois Company | Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ 0	[4]	$ 0

[1] Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s rail transportation surcharges for coal through December 2017. Losses deferred as current regulatory assets include $19 million at Ameren and Ameren Missouri.

[2] Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through March 2021 at Ameren and Ameren Missouri and through October 2019 at Ameren Illinois. Gains deferred as current regulatory liabilities include $1 million at Ameren and Ameren Missouri. Losses deferred as current regulatory assets include $31 million, $5 million, and $26 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.

[3] Represents net gains (losses) associated with power derivative contracts. These contracts are a partial hedge of power price requirements through May 2032 at Ameren and Ameren Illinois and through December 2016 at Ameren Missouri. Gains deferred as current regulatory liabilities include $22 million at Ameren and Ameren Missouri. Losses deferred as current regulatory assets include $13 million, $1 million, and $12 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.

[4] Represents net losses on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s uranium requirements through January 2017. Losses deferred as current regulatory assets include $1 million at Ameren and Ameren Missouri.