Derivative Financial Instruments (Derivatives That Qualify For Regulatory Deferral) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	$ 19		$ 26		$ 75		$ (149)
Ameren Missouri [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	28		(10)		22		(8)
Ameren Illinois Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(9)		104		53		(56)
Fuel Oils [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(4)		(19)		(4)		(14)
Fuel Oils [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(4)		(19)		(4)		(14)
Natural Gas [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(12)		46		24		28
Natural Gas [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(2)		5		2		3
Natural Gas [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(10)		41		22		25
Power [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	36	[1]	(1)	[1]	56	[1]	(163)	[1]
Power [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	35		4		25		3
Power [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	1		63		31		(81)
Uranium [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(1)				(1)
Uranium [Member] | Ameren Missouri [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	$ (1)				$ (1)

[1] Amounts include intercompany eliminations.