Derivative Financial Instruments (Cumulative Amount Of Pretax Net Gains (Losses) On All Derivative Instruments In OCI) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2013		Dec. 31, 2012
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 216		$ 100
Current losses deferred as regulatory assets	156		247
Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	23
Current losses deferred as regulatory assets	13
Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	3
Current losses deferred as regulatory assets	1
Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	2
Current losses deferred as regulatory assets	32
Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	5
Regulatory Liabilities Or Assets | Power
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(89)	[1]	(99)	[1]
Regulatory Liabilities Or Assets | Fuel Oils
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	2	[2]	4	[2]
Regulatory Liabilities Or Assets | Natural Gas
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(55)	[3]	(107)	[3]
Regulatory Liabilities Or Assets | Uranium
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(6)	[4]	(2)	[4]
Union Electric Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	57		18
Current losses deferred as regulatory assets	118		163
Union Electric Company | Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	23
Current losses deferred as regulatory assets	4
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 | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	5
Union Electric Company | Uranium
Derivative [Line Items]
Current losses deferred as regulatory assets	5
Union Electric Company | Regulatory Liabilities Or Assets | Power
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	19	[1]	12	[1]
Union Electric Company | Regulatory Liabilities Or Assets | Fuel Oils
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	2	[2]	4	[2]
Union Electric Company | Regulatory Liabilities Or Assets | Natural Gas
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(10)	[3]	(14)	[3]
Union Electric Company | Regulatory Liabilities Or Assets | Uranium
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(6)	[4]	(2)	[4]
Ameren Illinois Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	159		82
Current losses deferred as regulatory assets	38		84
Ameren Illinois Company | Power
Derivative [Line Items]
Current losses deferred as regulatory assets	9
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current losses deferred as regulatory assets	27
Ameren Illinois Company | Regulatory Liabilities Or Assets | Power
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(108)	[1]	(111)	[1]
Ameren Illinois Company | Regulatory Liabilities Or Assets | Fuel Oils
Derivative [Line Items]
Cumulative deferred pretax gains (losses)		[2]		[2]
Ameren Illinois Company | Regulatory Liabilities Or Assets | Natural Gas
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(45)	[3]	(93)	[3]
Ameren Illinois Company | Regulatory Liabilities Or Assets | Uranium
Derivative [Line Items]
Cumulative deferred pretax gains (losses)		[4]		[4]

[1] 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 December 31, 2013. Current gains deferred as regulatory liabilities include $23 million and $23 million at Ameren and Ameren Missouri, respectively, as of December 31, 2013. Current losses deferred as regulatory assets include $13 million, $4 million, and $9 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of December 31, 2013.

[2] 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 2016, as of December 31, 2013. Current gains deferred as regulatory liabilities include $3 million and $3 million at Ameren and Ameren Missouri as of December 31, 2013, respectively. Current losses deferred as regulatory assets include $1 million and $1 million at Ameren and Ameren Missouri as of December 31, 2013, respectively.

[3] 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, in each case as of December 31, 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 December 31, 2013. Current losses deferred as regulatory assets include $32 million, $5 million, and $27 million at Ameren, Ameren Missouri and Ameren Illinois, respectively, as of December 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 October 2016, as of December 31, 2013. Current losses deferred as regulatory assets include $5 million and $5 million at Ameren and Ameren Missouri as of December 31, 2013, respectively.