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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 1	$ 5	$ 5	$ 5
Included in regulatory assets/liabilities	1	(2)	(1)	(2)
Purchases				1
Settlements			(2)	(1)
Ending balance	2	3	2	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	1	(1)		(1)
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(5)	(2)	(6)	(2)
Included in regulatory assets/liabilities	(4)	(2)	(4)	(2)
Settlements	2	1	3	1
Ending balance	(7)	(3)	(7)	(3)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(4)	(1)	(4)	(1)
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
Purchases	1	(1)	(1)	0
Settlements	(1)		1
Ending balance	0	1	0	1
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	(110)	(79)	(89)	(100)
Included in regulatory assets/liabilities	3	2	(14)	21
Purchases	34	40	34	40
Settlements	(14)	(9)	(19)	(6)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3				(2)
Transfer out of Level 3	(1)	3		4
Ending balance	(88)	(43)	(88)	(43)
Change in unrealized gains (losses) related to assets/liabilities held at period end	14	(1)	(2)	15
Union Electric Company | Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1	5	5	5
Included in regulatory assets/liabilities	1	(2)	(1)	(2)
Purchases				1
Settlements			(2)	(1)
Ending balance	2	3	2	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	1	(1)		(1)
Union Electric Company | Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(5)	(2)	(6)	(2)
Included in regulatory assets/liabilities	(4)	(2)	(4)	(2)
Settlements	2	1	3	1
Ending balance	(7)	(3)	(7)	(3)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(4)	(1)	(4)	(1)
Union Electric Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0	0	0	0
Purchases	0	(1)	0	(1)
Ending balance	0	(1)	0	(1)
Change in unrealized gains (losses) related to assets/liabilities held at period end		(1)
Union Electric Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	10	2	19	11
Included in regulatory assets/liabilities	(13)	1	(18)	6
Purchases	34	40	34	40
Settlements	(15)	(9)	(20)	(22)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Liability, Transfers Into Level 3				(2)
Transfer out of Level 3	(1)	3		4
Ending balance	15	37	15	37
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)	3	(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
Purchases	1	0	(1)	1
Settlements	(1)		1
Ending balance	0	2	0	2
Change in unrealized gains (losses) related to assets/liabilities held at period end
Ameren Illinois Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(120)	(81)	(108)	(111)
Included in regulatory assets/liabilities	16	1	4	15
Settlements	1		1	16
Transfer out of Level 3
Ending balance	(103)	(80)	(103)	(80)
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ 15	$ (4)	$ 1	$ 15