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, 2013	Sep. 30, 2012		Sep. 30, 2013	Sep. 30, 2012
Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	$ 3	$ 3		$ 5	$ 3
Included in regulatory assets/liabilities	1	1		(1)	(1)
Total realized and unrealized gains (losses)	1	1		(1)	(1)
Purchases	1	2		2	4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(1)	(1)		(1)	(2)
Settlements	(1)			(2)	(1)
Transfers into Level 3					2
Ending balance	3	5		3	5
Change in unrealized gains (losses) related to assets/liabilities held at period end	1	2		0	(1)
Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(3)	(1)		(2)	(1)
Included in regulatory assets/liabilities		(1)		(2)	(1)
Total realized and unrealized gains (losses)		(1)		(2)	(1)
Purchases				(2)
Settlements	(2)			1
Ending balance	(5)	(2)		(5)	(2)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(2)	(1)		(2)	(1)
Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	1			0	(174)
Included in regulatory assets/liabilities	(2)			(1)	(28)
Total realized and unrealized gains (losses)	(2)			(1)	(28)
Purchases	1			1
Settlements					17
Transfer out of Level 3					185
Ending balance	0	0		0	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)				7
Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(43)	(81)	[1]	(100)	81
Included in regulatory assets/liabilities	(20)	(6)	[1]	1	(174)	[1]
Total realized and unrealized gains (losses)	(20)	(6)	[1]	1	(174)	[1]
Purchases				40	22
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	1	(1)	[1]	1	(1)
Settlements	(3)	(1)	[1]	(9)	(15)	[1]
Transfers into Level 3	1			(3)
Transfer out of Level 3		(2)	[1]	4	4
Ending balance	(66)	(91)	[1]	(66)	(91)	[1]
Change in unrealized gains (losses) related to assets/liabilities held at period end	(18)	(7)	[1]	(7)	(171)
Union Electric Company | Fuel Oils
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	3	3		5	3
Included in regulatory assets/liabilities	1	1		(1)	(1)
Total realized and unrealized gains (losses)	1	1		(1)	(1)
Purchases	1	2		2	4
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	(1)	(1)		(1)	(2)
Settlements	(1)			(2)	(1)
Transfers into Level 3					2
Ending balance	3	5		3	5
Change in unrealized gains (losses) related to assets/liabilities held at period end	1	2		0	(1)
Union Electric Company | Uranium
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(3)	(1)		(2)	(1)
Included in regulatory assets/liabilities		(1)		(2)	(1)
Total realized and unrealized gains (losses)		(1)		(2)	(1)
Purchases				(2)
Settlements	(2)			1
Ending balance	(5)	(2)		(5)	(2)
Change in unrealized gains (losses) related to assets/liabilities held at period end	(2)	(1)		(2)	(1)
Union Electric Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(1)			0	(14)
Included in regulatory assets/liabilities					(2)
Total realized and unrealized gains (losses)					(2)
Purchases	1
Settlements					(1)
Transfer out of Level 3					15
Ending balance	0	0		0	0
Change in unrealized gains (losses) related to assets/liabilities held at period end					7
Union Electric Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	37	26		11	21
Included in regulatory assets/liabilities	(3)	(4)		3	5
Total realized and unrealized gains (losses)	(3)	(4)		3	5
Purchases				40	22
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales	1	(1)		1	(1)
Settlements	(6)	(4)		(28)	28
Transfers into Level 3	1			(3)
Transfer out of Level 3		(2)		4	(4)
Ending balance	28	15		28	15
Change in unrealized gains (losses) related to assets/liabilities held at period end	(2)	(5)			(1)
Ameren Illinois Company | Natural Gas
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	2			0	(160)
Included in regulatory assets/liabilities	(2)			(1)	(26)
Total realized and unrealized gains (losses)	(2)			(1)	(26)
Purchases				1
Settlements					(16)
Transfer out of Level 3					170
Ending balance	0	0		0	0
Change in unrealized gains (losses) related to assets/liabilities held at period end	(1)				0
Ameren Illinois Company | Power
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning balance	(80)	(221)		(111)	(140)
Included in regulatory assets/liabilities	(17)	2		(2)	(219)
Total realized and unrealized gains (losses)	(17)	2		(2)	(219)
Purchases					0
Settlements	3	54		19	(194)
Transfers into Level 3
Transfer out of Level 3				0
Ending balance	(94)	(165)		(94)	(165)
Change in unrealized gains (losses) related to assets/liabilities held at period end	$ (16)	$ (2)		$ (7)	$ (187)	[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.