Retirement Plans - Information Pertaining to Activity in Supplemental Retirement Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in benefit obligation:
Benefit obligation at beginning of year	$ (26,184)	$ (26,798)
Acquisition	0	0
Service cost	(1,042)	(1,023)	$ (652)
Interest cost	(1,025)	(957)	(739)
Actuarial (loss)/gain	511	1,382
Benefits paid	1,095	1,212
Benefit obligation at end of year	(26,645)	(26,184)	(26,798)
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Fair value of plan assets at end of year	0	0	$ 0
Funded status	(26,645)	(26,184)
Unrecognized net obligation existing at January 1, 1986	7	7
Unrecognized net actuarial loss	4,782	5,843
Unrecognized prior service cost	(75)	(115)
Accumulated other comprehensive income	(4,714)	(5,735)
Accrued benefit cost	(26,645)	(26,184)
Accumulated benefit obligation	$ (25,241)	$ (24,469)