Employee Postretirement Benefits - Schedule of One-percentage-point Change in Assumed Health Care Cost (Detail) - Assumed One-percentage-point Change [Member] $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Effect on PBO - Increase	$ 7
Effect on PBO - Decrease	$ (6)

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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- Definition

Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (m)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- Definition

Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate.

+ References

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