Schedule of One Percentage-Point Change In Assumed Health Care Cost Trend Rate (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($)
Compensation And Retirement Disclosure [Abstract]
Total service and interest cost components, Increase	$ 41
Postretirement benefit obligation, Increase	783
Total service and interest cost components, Decrease	(34)
Postretirement benefit obligation, Decrease	$ (631)

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- Definition

Amount of decrease in accumulated postretirement benefit obligation from one-percentage-point decrease in assumed health care cost trend rate.

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

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- Definition

Amount of decrease in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point decrease in assumed health care cost trend rate.

+ References

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- Definition

Amount of increase in accumulated postretirement benefit obligation from one-percentage-point increase in assumed health care cost trend rate.

+ References

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- Definition

Amount of increase in service and interest cost components of net periodic postretirement benefit cost from one-percentage-point increase in assumed health care cost trend rate.

+ References

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