Retirement Plans and Other Postretirement Benefits - Components of net periodic benefit cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 155	$ 155	$ 311	$ 313
Interest cost	426	432	857	863
Expected return on plan assets	(709)	(637)	(1,264)	(1,128)
Amortization of net loss	343	287	629	578
Accretion of prior service cost	(59)	(59)	(118)	(118)
Net periodic benefit cost	156	178	415	508
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	25	20	46	41
Interest cost	27	29	56	59
Expected return on plan assets
Amortization of net loss	6	5	12	11
Accretion of prior service cost	(4)	(5)	(9)	(10)
Net periodic benefit cost	$ 54	$ 49	$ 105	$ 101