Components of Periodic Benefit Costs for Defined-Benefit Pension, Postretirement Healthcare and Supplemental Retirement Plan (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 254	$ 215
Interest cost	470	476
Expected return on plan assets	(559)	(509)
Amortization of net actuarial loss	564	473
Net periodic benefit cost	729	655
Postretirement Healthcare Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	9	8
Interest cost	15	18
Amortization of net actuarial loss	2	6
Net periodic benefit cost	26	32
Supplemental Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	12	7
Interest cost	39	40
Amortization of net actuarial loss	57	48
Net periodic benefit cost	$ 108	$ 95