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, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Beginning Balance	$ 4,755	$ 4,035
Total Unrealized (Losses) or Gains	0	0
Total Realized Gains or (Losses)	0	0
Purchases	675	1,805
Maturities/ Redemptions	(865)	(1,085)
Ending Balance	4,565	4,755
Certificates of Deposit [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Beginning Balance	4,755	4,035
Total Unrealized (Losses) or Gains	0	0
Total Realized Gains or (Losses)	0	0
Purchases	675	1,805
Maturities/ Redemptions	(865)	(1,085)
Ending Balance	$ 4,565	$ 4,755