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, 2014		Dec. 31, 2013
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 248		$ 216
Current losses deferred as regulatory assets	220		156
Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	2
Current losses deferred as regulatory assets	1
Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)		[1]	2
Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	6
Current losses deferred as regulatory assets	20
Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(30)	[2]	(55)
Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	15
Current losses deferred as regulatory assets	14
Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(111)	[3]	(89)
Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	4
Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(5)	[4]	(6)
Union Electric Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	47		57
Current losses deferred as regulatory assets	145		118
Union Electric Company | Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	2
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]	2
Union Electric Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	4
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	15
Current losses deferred as regulatory assets	6
Union Electric Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	9	[3]	19
Union Electric Company | Uranium
Derivative [Line Items]
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]	(6)
Ameren Illinois Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	201		159
Current losses deferred as regulatory assets	75		38
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	5
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)	(24)	[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)	$ (120)	[3]	$ (108)

[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 December 2016 as of March 31, 2014. Current gains deferred as regulatory liabilities include $2 million and $2 million at Ameren and Ameren Missouri, respectively, as of March 31, 2014. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri, respectively, as of March 31, 2014

[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 2017 at Ameren Illinois, in each case as of March 31, 2014. 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, 2014. Current losses deferred as regulatory assets include $20 million, $4 million, and $16 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of March 31, 2014.

[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, 2014. Current gains deferred as regulatory liabilities include $15 million and $15 million at Ameren and Ameren Missouri, respectively, as of March 31, 2014. Current losses deferred as regulatory assets include $14 million, $6 million, and $8 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of March 31, 2014.

[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, as of March 31, 2014. Current losses deferred as regulatory assets include $4 million and $4 million at Ameren and Ameren Missouri, respectively, as of March 31, 2014.