Derivative Financial Instruments (Derivatives That Qualify For Regulatory Deferral) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	$ 56	$ (175)
Union Electric Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(6)	2
Ameren Illinois Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	62	(160)
Fuel Oils [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral		5
Fuel Oils [Member] | Union Electric Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral		5
Natural Gas [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	36	(18)
Natural Gas [Member] | Union Electric Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	4	(2)
Natural Gas [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	32	(16)
Power [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	20	(162)
Power [Member] | Union Electric Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	(10)	(1)
Power [Member] | Ameren Illinois Company [Member]
Derivative [Line Items]
Net change in market value of derivatives that qualify for regulatory deferral	$ 30	$ (144)