Retirement Plans and Other Postretirement Benefits - Reconciliation of Beginning and Ending Balances for Measurements in Hierarchy Level 3 (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Beginning Balance	$ 4,565	$ 4,755
Total Unrealized (Losses) or Gains	0	0
Total Realized Gains or (Losses)	0	0
Purchases	535	675
Maturities/ Redemptions	(890)	(865)
Ending Balance	4,210	4,565
Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Beginning Balance	4,565	4,755
Total Unrealized (Losses) or Gains	0	0
Total Realized Gains or (Losses)	0	0
Purchases	535	675
Maturities/ Redemptions	(890)	(865)
Ending Balance	$ 4,210	$ 4,565