Retirement Plans - Information Pertaining to Activity in Supplemental Retirement Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in benefit obligation:
Benefit obligation at beginning of year	$ (28,472)	$ (26,645)
Service cost	(973)	(941)	$ (1,042)
Interest cost	(949)	(991)	(1,025)
Actuarial (loss)/gain	92	(1,203)
Plan amendments		185
Benefits paid	1,106	1,123
Benefit obligation at end of year	(29,196)	(28,472)	(26,645)
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	(29,196)	(28,472)
Unrecognized net obligation existing at January 1, 1986	3	4
Unrecognized net actuarial loss	4,993	5,596
Unrecognized prior service cost	(194)	(248)
Accumulated other comprehensive income	(4,802)	(5,352)
Accrued benefit cost	(29,196)	(28,472)
Accumulated benefit obligation	$ (27,544)	$ (26,432)