Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2013		Dec. 31, 2012
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 180		$ 100
Current losses deferred as regulatory assets	192		247
Fuel Oils [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	2
Current losses deferred as regulatory assets	1
Fuel Oils [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)		[1]	4	[1]
Natural Gas [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	2
Current losses deferred as regulatory assets	52
Natural Gas [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(83)	[2]	(107)	[2]
Power [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	44
Current losses deferred as regulatory assets	16
Power [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(43)	[3]	(99)	[3]
Uranium [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	3
Uranium [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(3)	[4]	(2)	[4]
Ameren Missouri [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	71		18
Current losses deferred as regulatory assets	132		163
Ameren Missouri [Member] | Fuel Oils [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	2
Current losses deferred as regulatory assets	1
Ameren Missouri [Member] | Fuel Oils [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)		[1]	4	[1]
Ameren Missouri [Member] | Natural Gas [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	7
Ameren Missouri [Member] | Natural Gas [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(12)	[2]	(14)	[2]
Ameren Missouri [Member] | Power [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	43
Current losses deferred as regulatory assets	6
Ameren Missouri [Member] | Power [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	37	[3]	12	[3]
Ameren Missouri [Member] | Uranium [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	3
Ameren Missouri [Member] | Uranium [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(3)	[4]	(2)	[4]
Ameren Illinois Company [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	110		82
Current losses deferred as regulatory assets	61		84
Ameren Illinois Company [Member] | Natural Gas [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	45
Ameren Illinois Company [Member] | Natural Gas [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(71)	[2]	(93)	[2]
Ameren Illinois Company [Member] | Power [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	10
Ameren Illinois Company [Member] | Power [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ (80)	[3]	$ (111)	[3]

[1] Represents net gains 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 June 30, 2013. Current gains deferred as regulatory liabilities include $2 million and $2 million at Ameren and Ameren Missouri, respectively, as of June 30, 2013. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri, respectively, as of June 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 October 2016 at Ameren Illinois as of June 30, 2013. Current gains deferred as regulatory liabilities include $2 million, $1 million, and $1 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of June 30, 2013. Current losses deferred as regulatory assets include $52 million, $7 million, and $45 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 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 June 30, 2013. Current gains deferred as regulatory liabilities include $44 million, $43 million, and $1 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively, as of June 30, 2013. Current losses deferred as regulatory assets include $16 million, $6 million, and $10 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of June 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 May 2015 as of June 30, 2013. Current losses deferred as regulatory assets include $3 million and $3 million at Ameren and Ameren Missouri, respectively, as of June 30, 2013.