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, 2014		Dec. 31, 2013
Derivative [Line Items]
Current gains deferred as regulatory liabilities	$ 106		$ 216
Current regulatory assets	295		156
Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	15
Current regulatory assets	14
Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	21
Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	2
Current regulatory assets	37
Uranium
Derivative [Line Items]
Current regulatory assets	2
Regulatory Liabilities Or Assets | Power
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(130)	[1]	(89)
Regulatory Liabilities Or Assets | Fuel Oils
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(29)	[2]	2
Regulatory Liabilities Or Assets | Natural Gas
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(54)	[3]	(55)
Regulatory Liabilities Or Assets | Uranium
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(2)	[4]	(6)
Union Electric Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	18		57
Current regulatory assets	163		118
Union Electric Company | Power
Derivative [Line Items]
Current gains deferred as regulatory liabilities	15
Current regulatory assets	3
Union Electric Company | Fuel Oils
Derivative [Line Items]
Current gains deferred as regulatory liabilities	21
Union Electric Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current regulatory assets	6
Union Electric Company | Uranium
Derivative [Line Items]
Current regulatory assets	2
Union Electric Company | Regulatory Liabilities Or Assets | Power
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	12	[1]	19
Union Electric Company | Regulatory Liabilities Or Assets | Fuel Oils
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(29)	[2]	2
Union Electric Company | Regulatory Liabilities Or Assets | Natural Gas
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(11)	[3]	(10)
Union Electric Company | Regulatory Liabilities Or Assets | Uranium
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(2)	[4]	(6)
Ameren Illinois Company
Derivative [Line Items]
Current gains deferred as regulatory liabilities	84		159
Current regulatory assets	129		38
Ameren Illinois Company | Power
Derivative [Line Items]
Current regulatory assets	11
Ameren Illinois Company | Natural Gas
Derivative [Line Items]
Current gains deferred as regulatory liabilities	1
Current regulatory assets	31
Ameren Illinois Company | Regulatory Liabilities Or Assets | Power
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	(142)	[1]	(108)
Ameren Illinois Company | Regulatory Liabilities Or Assets | Natural Gas
Derivative [Line Items]
Cumulative deferred pretax gains (losses)	$ (43)	[3]	$ (45)

[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. Current gains deferred as regulatory liabilities include $15 million and $15 million at Ameren and Ameren Missouri, respectively. Current losses deferred as regulatory assets include $14 million, $3 million, and $11 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.

[2] 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 $21 million and $21 million at Ameren and Ameren Missouri, 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 October 2018 at Ameren Illinois. Current gains deferred as regulatory liabilities include $2 million, $1 million, and $1 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively. Current losses deferred as regulatory assets include $37 million, $6 million, and $31 million at Ameren, Ameren Missouri, and Ameren Illinois, respectively.

[4] Represents net losses associated with 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 $2 million and $2 million at Ameren and Ameren Missouri, respectively.