Derivative Financial Instruments (Derivatives That Qualify For Regulatory Deferral) (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
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	$ (16)		$ 44		$ 59		$ (105)
Union Electric Company
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(11)		4		11		(4)
Ameren Illinois Company
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(5)		96		48		40
Fuel Oils
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	1		5		(3)		(9)
Fuel Oils | Union Electric Company
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	1		5		(3)		(9)
Natural Gas
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	9		46		33		74
Natural Gas | Union Electric Company
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral			6		2		9
Natural Gas | Ameren Illinois Company
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	9		40		31		65
Power
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(24)	[1]	(6)	[1]	32	[1]	(169)	[1]
Power | Union Electric Company
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(10)		(6)		15		(3)
Power | Ameren Illinois Company
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(14)		56		17		(25)
Uranium
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(2)		(1)		(3)		(1)
Uranium | Union Electric Company
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	$ (2)		$ (1)		$ (3)		$ (1)

[1] Amounts include intercompany eliminations.