Employee Benefit Plans - Obligation and Funded Status of Defined Benefit Pension Plan and Retirement Plan (Detail) - 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	$ 33,154	$ 31,510
Service cost	531	531	$ 785
Interest cost	1,329	1,367	1,350
Actuarial loss	2,449	1,132
Benefits paid	(1,461)	(1,386)
Ending benefit obligation	36,002	33,154	31,510
Change in plan assets
Fair value of plan assets at beginning of year	22,015	19,933
Return on assets	3,481	1,551
Employer contribution	1,611	1,917
Benefits paid	(1,461)	(1,386)
Fair value of plan assets at end of year	25,646	22,015	19,933
Funded status (underfunded)	(10,356)	(11,139)
Amounts recognized in the consolidated balance sheet
Current liabilities	560	544
Non-current liabilities	9,796	10,595
Unrecognized loss included in accumulated other comprehensive loss
Unrecognized loss before tax, beginning balance	9,220	8,919
Amortization	(810)	(762)
Liability loss	2,449	1,132
Asset gain	(1,951)	(69)
Unrecognized loss before tax, ending balance	8,908	9,220	$ 8,919
Tax impact	(3,309)	(3,425)
Unrecognized loss included in accumulated other comprehensive loss, net of tax	$ 5,599	$ 5,795