Postretirement Benefit Plans - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Compensation And Retirement Disclosure [Abstract]
Liability related to funded defined-benefit pension plans	$ 359	$ 353
Contributions to benefit plans	9
Insurance cash value and Rabbi Trust investments supporting supplemental pension plan liability	63	54
Unqualified supplemental retirement pension plan liability	$ 61	$ 59

The increase in the fair value of plan assets from contributions made by the employer.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (b)(3)

-URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration