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, 2015		Dec. 31, 2014
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 107		$ 106
Current losses deferred as regulatory assets	265		295
Fuel Oils
Derivative [Line Items]
Current losses deferred as regulatory assets	20
Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(27)	[1]	(29)
Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	38
Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(63)	[2]	(54)
Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	7
Current losses deferred as regulatory assets	12
Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(158)	[3]	(130)
Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	1
Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(1)	[4]	(2)
Union Electric Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	32		18
Current losses deferred as regulatory assets	158		163
Union Electric Company | Fuel Oils
Derivative [Line Items]
Current losses deferred as regulatory assets	20
Union Electric Company | Fuel Oils | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(27)	[1]	(29)
Union Electric Company | Natural Gas
Derivative [Line Items]
Current losses deferred as regulatory assets	6
Union Electric Company | Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(14)	[2]	(11)
Union Electric Company | Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	7
Current losses deferred as regulatory assets	1
Union Electric Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	6	[3]	12
Union Electric Company | Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	1
Union Electric Company | Uranium | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(1)	[4]	(2)
Ameren Illinois Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	69		84
Current losses deferred as regulatory assets	105		129
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	32
Ameren Illinois Company | Natural Gas | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(49)	[2]	(43)
Ameren Illinois Company | Power
Derivative [Line Items]
Current losses deferred as regulatory assets	11
Ameren Illinois Company | Power | Regulatory Liabilities Or Assets [Member]
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ (164)	[3]	$ (142)

[1] Represents net losses associated with fuel oils derivative contracts at Ameren Missouri. These contracts are a partial hedge of Ameren Missouri’s rail transportation surcharges for coal through December 2017. Current losses deferred as regulatory assets include $20 million at Ameren and Ameren Missouri.

[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 at Ameren and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $38 million, $6 million, and $32 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 2016 at Ameren Missouri. Current gains deferred as regulatory liabilities include $7 million at Ameren and Ameren Missouri. Current losses deferred as regulatory assets include $12 million, $1 million, and $11 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 $1 million at Ameren and Ameren Missouri.