Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 135		$ 100
Current losses deferred as regulatory assets	196		247
Fuel Oils [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	4
Current losses deferred as regulatory assets	1
Fuel Oils [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	4	[1]	4	[1]
Natural Gas [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	6
Current losses deferred as regulatory assets	45
Natural Gas [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(71)	[2]	(107)	[2]
Power [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	12
Current losses deferred as regulatory assets	21
Power [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(79)	[3]	(99)	[3]
Uranium [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	2
Uranium [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(2)	[4]	(2)	[4]
Union Electric Company [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	23		18
Current losses deferred as regulatory assets	144		163
Union Electric Company [Member] | Fuel Oils [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	4
Current losses deferred as regulatory assets	1
Union Electric Company [Member] | Fuel Oils [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	4	[1]	4	[1]
Union Electric Company [Member] | Natural Gas [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	7
Union Electric Company [Member] | Natural Gas [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(10)	[2]	(14)	[2]
Union Electric Company [Member] | Power [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	12
Current losses deferred as regulatory assets	10
Union Electric Company [Member] | Power [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	2	[3]	12	[3]
Union Electric Company [Member] | Uranium [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	2
Union Electric Company [Member] | Uranium [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(2)	[4]	(2)	[4]
Ameren Illinois Company [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	113		82
Current losses deferred as regulatory assets	52		84
Ameren Illinois Company [Member] | Natural Gas [Member]
Derivative [Line Items]
Current gains deferred as regulatory liabilities	5
Current losses deferred as regulatory assets	38
Ameren Illinois Company [Member] | Natural Gas [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(61)	[2]	(93)	[2]
Ameren Illinois Company [Member] | Power [Member]
Derivative [Line Items]
Current losses deferred as regulatory assets	11
Ameren Illinois Company [Member] | Power [Member] | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ (81)	[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 March 31, 2013. Current gains deferred as regulatory liabilities include $4 million and $4 million at Ameren and Ameren Missouri as of March 31, 2013, respectively. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri as of March 31, 2013, respectively.

[2] Represents net losses associated with natural gas derivative contracts. These contracts are a partial hedge of natural gas requirements through March 2017 at Ameren and Ameren Missouri and through October 2016 at Ameren Illinois, in each case as of March 31, 2013. Current gains deferred as regulatory liabilities include $6 million, $1 million, and $5 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of March 31, 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 March 31, 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, in each case as of March 31, 2013. Current gains deferred as regulatory liabilities include $12 million and $12 million at Ameren and Ameren Missouri, respectively, as of March 31, 2013. Current losses deferred as regulatory assets include $21 million, $10 million, and $11 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of March 31, 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 September 2014 as of March 31, 2013. Current losses deferred as regulatory assets include $2 million and $2 million at Ameren and Ameren Missouri as of March 31, 2013, respectively.