Segment and Related Information - Financial Data for Products and Services (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012
Financial data for Products and Services
Electric Utilities' Revenues	$ 3,885	$ 3,589
Retail [Member]
Financial data for Products and Services
Electric Utilities' Revenues	3,298	3,092
Wholesale [Member]
Financial data for Products and Services
Electric Utilities' Revenues	432	349
Other [Member]
Financial data for Products and Services
Electric Utilities' Revenues	$ 155	$ 148

Amount of revenue from external customers for the reportable segment. Such disclosure is presented if the amount is: (a) included in the measure of segment profit or loss reviewed by the chief operating decision maker or (b) otherwise regularly provided to the chief operating decision maker, even if not included in that measure of segment profit or loss.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 131

-Paragraph 27

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_RevenuesFromExternalCustomers
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration