Certain Information Regarding Health and Welfare Plan (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in Plan Assets:
Fair value of assets at end of year	$ 234,136
Post-Retirement Benefits
Change in benefit obligation:
Benefit obligation at beginning of year	16,294	$ 17,280
Service cost		5	$ 4
Interest cost	577	639	700
Actuarial loss (gain)	517	(673)
Premiums and claims paid	(1,039)	(957)
Benefit obligation at end of year	16,349	16,294	17,280
Change in Plan Assets:
Fair value of assets at beginning of year	0	0
Employer contribution	1,039	957
Premiums and claims paid	(1,039)	(957)
Fair value of assets at end of year	0	0	$ 0
Funded status (included in "Other liabilities")	(16,349)	(16,294)
Changes recognized in other comprehensive (loss) income for the year ended December 31:
Amortization of prior service cost	249	249
Amortization of actuarial gain	(274)	(326)
Net actuarial loss (gain) arising during the year	517	(673)
Total recognized in other comprehensive loss for the year (pre-tax)	492	(750)
Accumulated other comprehensive loss (pre-tax) not yet recognized in net periodic benefit cost at December 31:
Prior service cost	(1,034)	(1,283)
Actuarial loss, net	5,380	5,137
Total accumulated other comprehensive loss (pre-tax)	$ 4,346	$ 3,854