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Postretirement Benefits Other Than Pensions - Net Periodic Pension and Postretirement Benefit (Income) Expense (Detail) - Other Postretirement Benefit Plan [Member] - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]      
Interest cost $ 120 $ 127 $ 221
Amortization of net actuarial loss 242 255 445
Amortization of prior service credit (1,314) (1,314) (1,314)
Total net periodic pension (income) expense $ (952) $ (932) $ (648)