Employee Retirement Benefits - Defined Contribution Plan (Detail) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Matching contributions to 401(k) and Top Hat plans	$ 6	$ 6
Qualified Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension contributions	$ 5

Amount of payment for pension benefits. Excludes other postretirement benefits.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3536-108585

Name:	us-gaap_PensionContributions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration