Retirement Plans - Information Pertaining to Activity in Supplemental Retirement Plans (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	$ (26,645)	$ (26,184)
Acquisition	0	0
Service cost	(941)	(1,042)	$ (1,023)
Interest cost	(991)	(1,025)	(957)
Actuarial (loss)/gain	(1,203)	511
Plan amendments	185
Benefits paid	1,123	1,095
Benefit obligation at end of year	(28,472)	(26,645)	(26,184)
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	(28,472)	(26,645)
Unrecognized net obligation existing at January 1, 1986	4	7
Unrecognized net actuarial loss	5,596	4,782
Unrecognized prior service cost	(248)	(75)
Accumulated other comprehensive income	(5,352)	(4,714)
Accrued benefit cost	(28,472)	(26,645)
Accumulated benefit obligation	$ (26,432)	$ (25,241)