Schedule of Measurement Dates (Detail)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Benefit Obligation
Defined Benefit Plan Disclosure [Line Items]
Measurement Dates	Dec. 31, 2017	Dec. 31, 2016
Fair Value Of Plan Assets
Defined Benefit Plan Disclosure [Line Items]
Measurement Dates	Dec. 31, 2017	Dec. 31, 2016

Month-end measurement date applicable when an accounting policy election is made to measure plan assets and benefit obligations as of the month-end that is closest to the fiscal year-end, in CCYY-MM-DD format.

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

Name:	us-gaap_DefinedBenefitPlanAccountingPolicyElectionMeasurementDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration