Commitments And Contingencies (Pumped-Storage Hydroelectric Facility Breach) (Detail) (Union Electric Company [Member], USD $) In Millions, unless otherwise specified	Mar. 31, 2013
Union Electric Company [Member]
Loss Contingencies [Line Items]
Insurance settlements receivable	$ 68

Amount due in settlement of a claim for reimbursement from an insurance company when the Company has suffered a loss covered under an insurance policy.

No definition available.

Name:	us-gaap_InsuranceSettlementsReceivable
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant