Retirement Plans and Other Postretirement Benefits - Summary of Retirement Plans and Other Postretirement Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets:
Fair value of plan assets at beginning of year	$ 41,418
Fair value of plan assets at end of year	46,753	$ 41,418
Retirement Plans [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	47,389	49,810
Service cost	524	661	$ 756
Interest cost	1,927	2,071	1,953
Actuarial loss (gain)	3,169	413
Benefits paid	(2,645)	(2,400)
Settlements	0	3,166
Benefit obligation at end of year	50,364	47,389	49,810
Change in plan assets:
Fair value of plan assets at beginning of year	41,418	41,490
Actual return on plan assets	5,799	3,314
Settlements	0	3,166
Employer contribution and non-qualified benefit payments	2,181	2,180
Fair value of plan assets at end of year	46,753	41,418	41,490
Funded status	(3,611)	(5,971)
Unrecognized net actuarial loss	21,256	22,018
Unrecognized prior service costs	(464)	(746)
Net amount recognized	17,181	15,301
Other Post Retirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	2,968	2,834
Service cost	48	46	59
Interest cost	118	133	110
Actuarial loss (gain)	(409)	36
Benefits paid	(114)	(81)
Settlements	0	0
Benefit obligation at end of year	2,611	2,968	2,834
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Settlements	0	0
Employer contribution and non-qualified benefit payments	114	81
Fair value of plan assets at end of year	0	0	$ 0
Funded status	(2,611)	(2,968)
Unrecognized net actuarial loss	316	767
Unrecognized prior service costs	0	0
Net amount recognized	$ (2,295)	$ (2,201)