Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended
Jun. 30, 2013
Regulatory Assets [Abstract]
Reduction in net cash flow from financing activities instead of operating activities	$ 108

Amount of each individual error being corrected in the correcting adjustment.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 250

-SubTopic 10

-Section S99

-Paragraph 2

-Subparagraph (SAB TOPIC 1.N.Q3)

-URI http://asc.fasb.org/extlink&oid=26874127&loc=d3e30840-122693

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 1

-Section N

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