Note Employee benefits (Effects of changes in assumed health care cost trend rates) (Detail) - Postretirement Health Care Benefit Plan $ in Thousands	12 Months Ended
Dec. 31, 2017 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Effect on total service cost and interest cost components, one percentage point increase	$ 191
Effect on postretirement benefit obligation, one percentage point increase	5,328
Effect on total service cost and interest cost components, one percentage point decrease	(254)
Effect on postretirement benefit obligation, one percentage point decrease	$ (7,571)

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

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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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.

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

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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.

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