Benefit Plans - Amounts in Accumulated Other Comprehensive Loss Expected to be Recognized as Components of Net Periodic Pension Costs or Postretirement Benefits Costs in 2016 (Detail) $ in Thousands	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	$ 4,919
Prior service cost (credit)	(1,709)
Total expected to be recognized	3,210
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	3,681
Prior service cost (credit)	25
Total expected to be recognized	3,706
Postretirement Medical [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss	1,238
Prior service cost (credit)	(1,734)
Total expected to be recognized	$ (496)