Fair Value Measurements (Schedule Of Changes In The Fair Value Of Financial Assets And Liabilities Classified As Level Three In The Fair Value Hierarchy) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 2	$ 3	$ 5	$ 5
Included in regulatory assets/liabilities	(2)	1	(3)	(1)
Purchases		1		2
Sales		(1)		(1)
Settlements		(1)	(2)	(2)
Ending balance	0	3	0	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	(2)	1	(2)	0
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(7)	(3)	(6)	(2)
Included in regulatory assets/liabilities	3		(1)	(2)
Purchases		(2)		(2)
Settlements	1		4	1
Ending balance	(3)	(5)	(3)	(5)
Change in unrealized gains (losses) related to assets/liabilities held at period end	3	(2)	0	(2)
Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	1	0	0
Included in regulatory assets/liabilities	1	(2)	1	(1)
Purchases		1	(1)	1
Settlements			1
Ending balance	1	0	1	0
Change in unrealized gains (losses) related to assets/liabilities held at period end		(1)
Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(88)	(43)	(89)	(100)
Included in regulatory assets/liabilities	(28)	(20)	(42)	1
Purchases			34	40
Sales		1		1
Settlements	(3)	(3)	(22)	(9)
Transfers into Level 3		(1)		(3)
Transfer out of Level 3				4
Ending balance	(119)	(66)	(119)	(66)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(28)	(18)	(24)	(7)
Union Electric Company | Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2	3	5	5
Included in regulatory assets/liabilities	(2)	1	(3)	(1)
Purchases		1		2
Sales		(1)		(1)
Settlements		(1)	(2)	(2)
Ending balance	0	3	0	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	(2)	1	(2)	0
Union Electric Company | Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(7)	(3)	(6)	(2)
Included in regulatory assets/liabilities	3		(1)	(2)
Purchases		(2)		(2)
Settlements	1		4	1
Ending balance	(3)	(5)	(3)	(5)
Change in unrealized gains (losses) related to assets/liabilities held at period end	3	(2)	0	(2)
Union Electric Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	(1)	0	0
Included in regulatory assets/liabilities	0
Purchases		1	0	0
Ending balance	0	0	0	0
Change in unrealized gains (losses) related to assets/liabilities held at period end
Union Electric Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	15	37	19	11
Included in regulatory assets/liabilities	(5)	(3)	(23)	3
Purchases			34	40
Sales		1		1
Settlements	(5)	(6)	(25)	(28)
Transfers into Level 3		(1)		(3)
Transfer out of Level 3				4
Ending balance	5	28	5	28
Change in unrealized gains (losses) related to assets/liabilities held at period end	(6)	(2)	(3)
Ameren Illinois Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	2	0	0
Included in regulatory assets/liabilities	1	(2)	1	(1)
Purchases		0	(1)	1
Settlements			1
Ending balance	1	0	1	0
Change in unrealized gains (losses) related to assets/liabilities held at period end		(1)
Ameren Illinois Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(103)	(80)	(108)	(111)
Included in regulatory assets/liabilities	(23)	(17)	(19)	(2)
Settlements	2	3	3	19
Transfers into Level 3
Ending balance	(124)	(94)	(124)	(94)
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (22)	$ (16)	$ (21)	$ (7)