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, 2013	Jun. 30, 2012		Mar. 31, 2012		Jun. 30, 2013	Jun. 30, 2012
Fuel Oils [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 5	$ 7		$ 3		$ 5	$ 3
Included in regulatory assets/liabilities	(2)	(4)				(2)	(2)
Total realized and unrealized gains (losses)	(2)	(4)				(2)	(2)
Purchases		2				1	2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		(1)					(1)
Settlements		(1)				(1)	(1)
Transfers into Level 3							2
Ending balance	3	3				3	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)	(2)				(1)	(1)
Uranium [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2)	(1)		(1)		(2)	(1)
Included in regulatory assets/liabilities	(2)					(2)
Total realized and unrealized gains (losses)	(2)	0				(2)	0
Settlements	1					1
Ending balance	(3)	(1)		(1)		(3)	(1)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)					(1)
Natural Gas [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2			(174)		0	(174)
Included in regulatory assets/liabilities						1	(28)
Total realized and unrealized gains (losses)						1	(28)
Purchases	(1)
Settlements							17
Transfer out of Level 3							185
Ending balance	1					1
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)						123
Power [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(79)	(82)	[1]	81	[1]	(100)	81	[1]
Included in regulatory assets/liabilities	2	(10)	[1]			21	(168)	[1]
Total realized and unrealized gains (losses)	2	(10)	[1]			21	(168)	[1]
Purchases	40	22	[1]			40	22	[1]
Settlements	(9)	(10)	[1]			(6)	(14)	[1]
Transfers into Level 3						(2)
Transfer out of Level 3	3	(1)	[1]			4	(2)	[1]
Ending balance	(43)	(81)	[1]			(43)	(81)	[1]
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)	5	[1]			15	(179)	[1]
Ameren Missouri [Member] | Fuel Oils [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	5	7		3		5	3
Included in regulatory assets/liabilities	(2)	(4)				(2)	(2)
Total realized and unrealized gains (losses)	(2)	(4)				(2)	(2)
Purchases		2				1	2
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales		(1)					(1)
Settlements		(1)				(1)	(1)
Transfers into Level 3							2
Ending balance	3	3				3	3
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)	(2)				(1)	(1)
Ameren Missouri [Member] | Uranium [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(2)	(1)		(1)		(2)	(1)
Included in regulatory assets/liabilities	(2)					(2)
Total realized and unrealized gains (losses)	(2)	0				(2)	0
Settlements	1					1
Ending balance	(3)	(1)		(1)		(3)	(1)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)					(1)
Ameren Missouri [Member] | Natural Gas [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	0			(14)		0	(14)
Included in regulatory assets/liabilities							(2)
Total realized and unrealized gains (losses)							(2)
Purchases	(1)					(1)
Settlements							1
Transfer out of Level 3							15
Ending balance	(1)					(1)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)						9
Ameren Missouri [Member] | Power [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2	20		21		11	21
Included in regulatory assets/liabilities	1	(4)				6	9
Total realized and unrealized gains (losses)	1	(4)				6	9
Purchases	40	22				40	22
Settlements	(9)	(11)				(22)	(24)
Transfers into Level 3						(2)
Transfer out of Level 3	3	(1)				4	(2)
Ending balance	37	26				37	26
Change in unrealized gains (losses) related to assets/liabilities held at period end	3	(1)					3
Ameren Illinois Company [Member] | Natural Gas [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2			(160)		0	(160)
Included in regulatory assets/liabilities						1	(26)
Total realized and unrealized gains (losses)	0					1	(26)
Purchases						1
Settlements							16
Transfer out of Level 3							170
Ending balance	2					2
Change in unrealized gains (losses) related to assets/liabilities held at period end							114
Ameren Illinois Company [Member] | Power [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(81)	(284)		(140)		(111)	(140)
Included in regulatory assets/liabilities	1	(1)				15	(221)
Total realized and unrealized gains (losses)	1	(1)				15	(221)
Purchases
Settlements		64				16	140
Transfer out of Level 3
Ending balance	(80)	(221)		(284)		(80)	(221)
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (4)	$ (6)				$ 15	$ (195)	[2]

[1] Ameren amounts include the elimination of financial power contracts between Ameren Illinois and Marketing Company.

[2] The change in unrealized losses was due to decreases in long-term power prices applied to 20-year Ameren Illinois’ swap contracts, which expire May 2032.