Retirement Plans and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 155	$ 156	$ 466	$ 469
Interest cost	427	431	1,284	1,294
Expected return on plan assets	(630)	(564)	(1,894)	(1,692)
Amortization of net loss	314	287	943	865
Accretion of prior service cost	(58)	(58)	(176)	(176)
Net periodic benefit cost	208	252	623	760
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	23	21	69	62
Interest cost	30	29	86	88
Expected return on plan assets
Amortization of net loss	6	6	18	17
Accretion of prior service cost	(6)	(6)	(15)	(16)
Net periodic benefit cost	$ 53	$ 50	$ 158	$ 151