Retirement Plans and Other Postretirement Benefits - Summary of Retirement Plans and Other Postretirement Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Change in plan assets:
Fair value of plan assets at beginning of year	$ 41,437
Fair value of plan assets at end of year	41,490	$ 41,437
Retirement Plans [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	51,390	39,593
Service cost	756	528	$ 621
Interest cost	1,953	1,900	1,712
Actuarial loss (gain)	(1,915)	11,462
Benefits paid	(2,374)	(2,093)
Benefit obligation at end of year	49,810	51,390	39,593
Change in plan assets:
Fair value of plan assets at beginning of year	41,437	40,547
Actual return on plan assets	246	2,821
Benefits paid	(2,374)	(2,093)
Employer contribution and non-qualified benefit payments	2,181	162
Fair value of plan assets at end of year	41,490	41,437	40,547
Funded status	(8,320)	(9,953)
Unrecognized net actuarial loss	24,628	25,010
Unrecognized prior service costs	(1,029)	(1,309)
Net amount recognized	15,279	13,748
Other Post Retirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	2,896	2,330
Service cost	59	75	86
Interest cost	110	128	118
Actuarial loss (gain)	(141)	455
Benefits paid	(90)	(92)
Benefit obligation at end of year	2,834	2,896	2,330
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Benefits paid	(90)	(92)
Employer contribution and non-qualified benefit payments	90	92
Fair value of plan assets at end of year	0	0	$ 0
Funded status	(2,834)	(2,896)
Unrecognized net actuarial loss	756	950
Unrecognized prior service costs	0	0
Net amount recognized	$ (2,078)	$ (1,946)