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Schedule of One Percent Point Change in assumed Health Care Cost Trends Rate (Detail)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Defined Benefit Plan Disclosure  
Effect of one percentage point increase on total service and interest cost components $ 3
Effect of one percentage point increase on postretirement benefit obligations 64
Effect of one percentage point decrease on total service and interest cost components (3)
Effect of one percentage point decrease on postretirement benefit obligations $ (60)