Commitments And Contingencies (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
	Jun. 30, 2012 Year mwh cf
Commitments And Contingencies [Line Items]
Purchase commitment, MWh	48,000
Purchase commitment	$ 15
Agreement to purchase cubic feet of synthetic natural gas	15,500,000,000
Synthetic natural gas agreement in years	10
Ameren Illinois Company [Member]
Commitments And Contingencies [Line Items]
Purchase commitment, MWh	612,000
Purchase commitment	$ 17