Employee Benefit Plans (Accumulated Benefit Obligation in Excess of Plan Assets) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
General Discussion Of Pension And Other Postretirement Benefits [Abstract]
Accumulated benefit obligation	$ 7,392	$ 5,987
Fair value of plan assets	$ 6,103	$ 4,653

Amount of accumulated benefit obligation for defined benefit pension plan with accumulated benefit obligation in excess of plan assets.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2417-114920

Name:	us-gaap_DefinedBenefitPlanPensionPlansWithAccumulatedBenefitObligationsInExcessOfPlanAssetsAggregateAccumulatedBenefitObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant