Benefit Plans - Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost
Interest cost	323	336
Expected return on plan assets	(413)	(404)
Amortization of prior service cost
Amortization of net loss (gain)	90	68
Net periodic benefit cost
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	21	24
Interest cost	67	70
Expected return on plan assets
Amortization of prior service cost
Amortization of net loss (gain)	58	55
Net periodic benefit cost	$ 146	$ 149