Pension and Other Benefit Plans - Defined Benefit Plan, Amounts That Will be Amortized from Accumulated Other Comprehensive Income (loss) in Next Fiscal Year (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Postretirement medical plan
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Prior service cost (credit)	$ (28)
US Plan Assets
Pension Plans Postretirement And Other Employee Benefits [Line Items]
Net actuarial losses	209
Prior service cost (credit)	$ 23

Amount included in accumulated other comprehensive (income) loss for prior service cost (credit) expected to be recognized in net periodic benefit cost (credit) for fiscal year following most recent annual statement of financial position.

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

Name:	us-gaap_DefinedBenefitPlanExpectedAmortizationOfPriorServiceCostCreditNextFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant