Retirement Plans and Other Postretirement Benefits - Summary of Retirement Plans and Other Postretirement Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in plan assets:
Fair value of plan assets at beginning of year	$ 41,490
Fair value of plan assets at end of year	41,418	$ 41,490
Retirement Plans [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	49,810	51,390
Service cost	661	756	$ 528
Interest cost	2,071	1,953	1,900
Actuarial loss (gain)	413	(1,915)
Benefits paid	(2,400)	(2,374)
Settlements	(3,166)	0
Benefit obligation at end of year	47,389	49,810	51,390
Change in plan assets:
Fair value of plan assets at beginning of year	41,490	41,437
Actual return on plan assets	3,314	246
Benefits paid	(2,400)	(2,374)
Settlements	(3,166)	0
Employer contribution and non-qualified benefit payments	2,180	2,181
Fair value of plan assets at end of year	41,418	41,490	41,437
Funded status	(5,971)	(8,320)
Unrecognized net actuarial loss	22,018	24,628
Unrecognized prior service costs	(746)	(1,029)
Net amount recognized	15,301	15,279
Other Post Retirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	2,834	2,896
Service cost	46	59	75
Interest cost	133	110	128
Actuarial loss (gain)	36	(141)
Benefits paid	(81)	(90)
Settlements	0	0
Benefit obligation at end of year	2,968	2,834	2,896
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Benefits paid	(81)	(90)
Settlements	0	0
Employer contribution and non-qualified benefit payments	81	90
Fair value of plan assets at end of year	0	0	$ 0
Funded status	(2,968)	(2,834)
Unrecognized net actuarial loss	767	756
Unrecognized prior service costs	0	0
Net amount recognized	$ (2,201)	$ (2,078)