Retirement Plans and Other Postretirement Benefits - Summary of Retirement Plans and Other Postretirement Benefits (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in plan assets:
Fair value of plan assets at end of year	$ 41,437	$ 40,547
Retirement Plans [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	39,593	42,879
Service cost	528	621	622
Interest cost	1,900	1,712	1,726
Actuarial loss (gain)	11,462	(3,818)
Benefits paid	(2,093)	(1,801)
Benefit obligation at end of year	51,390	39,593	42,879
Change in plan assets:
Fair value of plan assets at beginning of year	40,547	34,420
Actual return on plan assets	2,821	5,764
Benefits paid	(2,093)	(1,801)
Employer contribution and non-qualified benefit payments	162	2,164
Fair value of plan assets at end of year	41,437	40,547	34,420
Funded status	(9,953)	954
Unrecognized net actuarial loss	25,010	14,138
Unrecognized prior service costs	(1,309)	(1,590)
Net amount recognized	13,748	13,502
Other Post Retirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	2,330	2,518
Service cost	75	86	82
Interest cost	128	118	116
Actuarial loss (gain)	455	(313)
Benefits paid	(92)	(79)
Benefit obligation at end of year	2,896	2,330	2,518
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Benefits paid	(92)	(79)
Employer contribution and non-qualified benefit payments	92	79
Fair value of plan assets at end of year	0	0	0
Funded status	(2,896)	(2,330)
Unrecognized net actuarial loss	950	510
Unrecognized prior service costs	0	(7)
Net amount recognized	$ (1,946)	$ (1,827)