Pension Plans - Funded Status and Accrued Pension Cost (Detail) - Pension Plan [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
CHANGE IN BENEFIT OBLIGATION:
Benefit obligation at beginning of year	$ 85,360	$ 87,236
Interest cost	3,019	3,173	$ 3,423
Actuarial loss	3,624	304
Benefits paid	(6,308)	(5,353)
Benefit obligation at end of year	85,695	85,360	87,236
CHANGE IN PLAN ASSETS:
Fair value of plan assets at beginning of year	65,446	65,439
Actual return on plan assets	10,107	5,360
Benefits paid	(6,308)	(5,353)
Fair value of plan assets at end of year	69,245	65,446	$ 65,439
Funded status and accrued pension cost	$ (16,450)	$ (19,914)