Postretirement Benefits Other Than Pension - Net Periodic Pension and Postretirement Benefit (Income) Expense (Detail) - Other Postretirement Benefit Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 108	$ 120	$ 127
Amortization of net actuarial loss	245	242	255
Amortization of prior service credit	(1,314)	(1,314)	(1,314)
Total net periodic pension (income) expense	$ (961)	$ (952)	$ (932)