Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Detail) (USD $) In Millions, unless otherwise specified	Sep. 30, 2014		Dec. 31, 2013
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 132		$ 216
Current losses deferred as regulatory assets	199		156
Fuel Oils
Derivative [Line Items]
Current losses deferred as regulatory assets	4
Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(5)	[1]	2
Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	19
Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(26)	[2]	(55)
Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	10
Current losses deferred as regulatory assets	14
Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(119)	[3]	(89)
Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	2
Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(3)	[4]	(6)
Union Electric Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	11		57
Current losses deferred as regulatory assets	137		118
Union Electric Company | Fuel Oils
Derivative [Line Items]
Current losses deferred as regulatory assets	4
Union Electric Company | Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(5)	[1]	2
Union Electric Company | Natural Gas
Derivative [Line Items]
Current losses deferred as regulatory assets	3
Union Electric Company | Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(6)	[2]	(10)
Union Electric Company | Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	10
Current losses deferred as regulatory assets	6
Union Electric Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	5	[3]	19
Union Electric Company | Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	2
Union Electric Company | Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(3)	[4]	(6)
Ameren Illinois Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	121		159
Current losses deferred as regulatory assets	62		38
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	16
Ameren Illinois Company | Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(20)	[2]	(45)
Ameren Illinois Company | Power
Derivative [Line Items]
Current losses deferred as regulatory assets	8
Ameren Illinois Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ (124)	[3]	$ (108)

[1] Represents net losses associated with fuel oil derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s transportation costs for coal through December 2017. Current losses deferred as regulatory assets include $4 million and $4 million at Ameren and Ameren Missouri, respectively.

[2] Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through October 2019 at Ameren and Ameren Missouri and through October 2018 at Ameren Illinois. Current gains deferred as regulatory liabilities include $1 million and $1 million at Ameren and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $19 million, $3 million, and $16 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 2015 at Ameren Missouri. Current gains deferred as regulatory liabilities include $10 million and $10 million at Ameren and Ameren Missouri. Current losses deferred as regulatory assets include $14 million, $6 million, and $8 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 December 2016. Current losses deferred as regulatory assets include $2 million and $2 million at Ameren and Ameren Missouri, respectively.