Pension and Employee Benefit Plans - Components of Pension and Employee Benefit Expenses (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 101		$ 214
Interest cost	$ 298	329	$ 595	657
Expected return on plan assets	(378)	(431)	(756)	(861)
Amortization of net actuarial loss	59	57	119	113
Net periodic benefit cost (gain)	(21)	56	(42)	123
Health Care Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	1	1	1	2
Amortization of loss	2	2	4	4
Net periodic benefit cost (gain)	$ 3	$ 3	$ 5	$ 6