Impairment and Other Charges (Summary Of Goodwill And Other Asset Impairment Pretax Charges) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Impairment and Other Charges [Line Items]
Impairment charge on long-lived assets and related charges	$ 2,578	[1]	$ 123	[1]	$ 101	[1]
Impairment charge on goodwill	0	[1]	0	[1]	420	[1]
Impairment charge on emission allowances	0	[1]	2	[1]	68	[1]
Total impairment charge	2,578	[1]	125	[1]	589	[1]
Union Electric Company [Member]
Impairment and Other Charges [Line Items]
Impairment charge on long-lived assets and related charges			89
Impairment charge on goodwill			0
Impairment charge on emission allowances			0
Disallowed capitalized costs associated with rebuilt Taum Sauk energy center			$ 89

[1] Includes amounts for registrant and nonregistrant subsidiaries.