Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level 3 In The Fair Value Hierarchy) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 5	$ 5
Included in regulatory assets/liabilities	(9)
Purchases		3
Sales		(1)
Settlements	(2)	(2)
Ending balance	(6)	5
Change in unrealized gains (losses) related to assets/liabilities still held	(6)
Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Included in regulatory assets/liabilities	1	(1)
Purchases	(2)	1
Sales	(1)
Settlements	1
Ending balance	(1)	0
Change in unrealized gains (losses) related to assets/liabilities still held	2
Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(89)	(100)
Included in regulatory assets/liabilities	(53)	(15)
Purchases	34	40
Sales	(1)
Settlements	(24)	(15)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		(3)
Transfers out of Level 3		4
Ending balance	(133)	(89)
Change in unrealized gains (losses) related to assets/liabilities still held	(43)	(25)
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(6)	(2)
Included in regulatory assets/liabilities	(1)	(3)
Purchases		(2)
Settlements	5	1
Ending balance	(2)	(6)
Change in unrealized gains (losses) related to assets/liabilities still held	(1)	(2)
Union Electric Company | Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	5	5
Included in regulatory assets/liabilities	(9)
Purchases		3
Sales		(1)
Settlements	(2)	(2)
Ending balance	(6)	5
Change in unrealized gains (losses) related to assets/liabilities still held	(6)
Union Electric Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Purchases		0
Sales	(1)
Ending balance	(1)	0
Union Electric Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	19	11
Included in regulatory assets/liabilities	(14)	3
Purchases	34	40
Sales	(1)
Settlements	(29)	(36)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3		(3)
Transfers out of Level 3		4
Ending balance	9	19
Change in unrealized gains (losses) related to assets/liabilities still held		(1)
Union Electric Company | Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(6)	(2)
Included in regulatory assets/liabilities	(1)	(3)
Purchases		(2)
Settlements	5	1
Ending balance	(2)	(6)
Change in unrealized gains (losses) related to assets/liabilities still held	(1)	(2)
Ameren Illinois Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0
Included in regulatory assets/liabilities	1	(1)
Purchases	(2)	1
Settlements	1
Ending balance	0	0
Change in unrealized gains (losses) related to assets/liabilities still held	2
Ameren Illinois Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(108)	(111)
Included in regulatory assets/liabilities	(39)	(18)
Settlements	5	21
Ending balance	(142)	(108)
Change in unrealized gains (losses) related to assets/liabilities still held	$ (43)	$ (24)