Derivative Financial Instruments (Derivatives That Qualify For Regulatory Deferral) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2012		Jun. 30, 2011		Jun. 30, 2012		Jun. 30, 2011
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	$ 26	[1]	$ 75	[1]	$ (149)	[1]	$ 136	[1]
Ameren Missouri [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(10)		8		(8)		39
Ameren Illinois Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	104		123		(56)		178
Fuel Oils [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(19)	[1]	(13)	[1]	(14)	[1]	16	[1]
Fuel Oils [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(19)		(13)		(14)		16
Natural Gas [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	46	[1]	3	[1]	28	[1]	34	[1]
Natural Gas [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	5		1		3		4
Natural Gas [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	41		2		25		30
Power [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(1)	[1]	88	[1]	(163)	[1]	90	[1]
Power [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	4		23		3		23
Power [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	63		121		(81)		148
Uranium [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral			(3)	[1]			(4)	[1]
Uranium [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral			$ (3)				$ (4)

[1] Includes amounts for intercompany eliminations.