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, 2013		Dec. 31, 2012
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 149		$ 100
Current losses deferred as regulatory assets	173		247
Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	3
Current losses deferred as regulatory assets	1
Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	1	[1]	4	[1]
Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	45
Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(74)	[2]	(107)	[2]
Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	29
Current losses deferred as regulatory assets	13
Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(67)	[3]	(99)	[3]
Uranium
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	4
Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(5)	[4]	(2)	[4]
Union Electric Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	61		18
Current losses deferred as regulatory assets	124		163
Union Electric Company | Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	3
Current losses deferred as regulatory assets	1
Union Electric Company | Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	1	[1]	4	[1]
Union Electric Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	7
Union Electric Company | Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(12)	[2]	(14)	[2]
Union Electric Company | Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	29
Current losses deferred as regulatory assets	3
Union Electric Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	27	[3]	12	[3]
Union Electric Company | Uranium
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	4
Union Electric Company | Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(5)	[4]	(2)	[4]
Ameren Illinois Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	88		82
Current losses deferred as regulatory assets	49		84
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Current losses deferred as regulatory assets	38
Ameren Illinois Company | Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(62)	[2]	(93)	[2]
Ameren Illinois Company | Power
Derivative [Line Items]
Current losses deferred as regulatory assets	10
Ameren Illinois Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ (94)	[3]	$ (111)	[3]

[1] Represents net gains (losses) on fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s transportation costs for coal through October 2015 as of September 30, 2013. Current gains deferred as regulatory liabilities include $3 million and $3 million at Ameren and Ameren Missouri, respectively, as of September 30, 2013. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri, respectively, as of September 30, 2013

[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 March 2017 at Ameren Illinois as of September 30, 2013. Current gains deferred as regulatory liabilities include $1 million, and $1 million at Ameren and Ameren Missouri, respectively, as of September 30, 2013. Current losses deferred as regulatory assets include $45 million, $7 million, and $38 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of September 30, 2013.

[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 as of September 30, 2013. Current gains deferred as regulatory liabilities include $29 million and $29 million at Ameren and Ameren Missouri, respectively, as of September 30, 2013. Current losses deferred as regulatory assets include $13 million, $3 million, and $10 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of September 30, 2013.

[4] Represents net losses on uranium derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s uranium requirements through October 2016 as of September 30, 2013. Current gains deferred as regulatory liabilities included $1 million and $1 million at Ameren and Ameren Missouri, respectively, as of September 30, 2013. Current losses deferred as regulatory assets include $4 million and $4 million at Ameren and Ameren Missouri, respectively, as of September 30, 2013.