Pension Plans, Postretirement and Other Employee Benefits - Estimated Subsidies Under Medicare Prescription Drug Improvement and Modernization Act (Detail) $ in Millions	Dec. 31, 2017 USD ($)
Retirement Benefits [Abstract]
2018	$ 9
2019	9
2020	9
2021	9
2022	9
2023-2027	$ 45

No definition available.

Name:	us-gaap_CompensationAndRetirementDisclosureAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of prescription drug subsidy receipts expected after fifth fiscal year following latest fiscal year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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

- Definition

Amount of prescription drug subsidy receipts expected in next fiscal year following latest fiscal year.

+ 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 (f)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 60
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=6414203&loc=d3e39716-114964

+ Details

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

- Definition

Amount of prescription drug subsidy receipts expected in fifth fiscal year following latest fiscal year.

+ References

+ Details

Name:	us-gaap_PrescriptionDrugSubsidyReceiptsYearFive1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of prescription drug subsidy receipts expected in fourth fiscal year following latest fiscal year.

+ References

+ Details

Name:	us-gaap_PrescriptionDrugSubsidyReceiptsYearFour1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of prescription drug subsidy receipts expected in third fiscal year following latest fiscal year.

+ References

+ Details

Name:	us-gaap_PrescriptionDrugSubsidyReceiptsYearThree1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of prescription drug subsidy receipts expected in second fiscal year following latest fiscal year.

+ References

+ Details

Name:	us-gaap_PrescriptionDrugSubsidyReceiptsYearTwo1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant