Postretirement Benefit Plans - Fair Value Changes Within Asset Categories for which Fair Value Measurements Use Significant Unobservable Inputs Level 3 (Detail) - Insurance Contracts [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items]
Beginning balance	$ 1,759	$ 1,969
Sale proceeds (benefit payments)	(134)	(144)
Change in unrealized gain	(84)	265
Foreign exchange impact	157	(331)
Ending balance	$ 1,698	$ 1,759