Discontinued Operations - (Loss) Income Recognized from Cross-Border Energy Lease Investments (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2014	Mar. 31, 2013
Discontinued Operations And Disposal Groups [Abstract]
Income tax expense, income from operations of discontinued operations	$ 0	$ 1
Income tax expense, Net gains associated with the accelerated disposition of retail electric and natural gas contracts	$ 0	$ 48

Income tax expense or benefit attributable to Income or Loss from discontinued operations during the phase-out period.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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