Fair Value Measurements (Schedule Of Transfers Between Fair Value Hierarchy Levels) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended					9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended
	Sep. 30, 2013	Sep. 30, 2013 Fuel Oils Transfer Into Or Out Of Level One [Member]	Mar. 31, 2012 Fuel Oils Transfer Into Or Out Of Level One [Member]	Sep. 30, 2013 Natural Gas Transfer Into/Out of Level 2	Sep. 30, 2013 Power	Sep. 30, 2013 Power	Sep. 30, 2013 Power Transfer Into/Out of Level 2	Sep. 30, 2013 Power Transfer Into/Out of Level 2	Sep. 30, 2013 Union Electric Company Power	Sep. 30, 2013 Union Electric Company Power	Sep. 30, 2013 Union Electric Company Power Transfer Into/Out of Level 2	Sep. 30, 2013 Union Electric Company Power Transfer Into/Out of Level 2	Sep. 30, 2013 Ameren Illinois Company Power
Derivative [Line Items]
Assets Transfers into Level 3					$ (1)	$ (3)	$ (1)	$ (3)	$ (1)	$ (3)	$ (1)	$ (3)
Assets Transfers out of Level 3						4		4		4		4
Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis Asset Liability Transfers, Net	$ (1)