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				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 254		$ 216		$ 508		$ 431
Interest cost	471		477		941		953
Expected return on plan assets	(559)		(509)		(1,118)		(1,018)
Amortization of net actuarial loss	564	[1]	470	[1]	1,128	[1]	943	[1]
Net periodic benefit cost	730		654		1,459		1,309
Postretirement Healthcare Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	10		10		19		18
Interest cost	14		18		29		36
Expected return on plan assets
Amortization of net actuarial loss	2	[1]	5	[1]	4	[1]	11	[1]
Net periodic benefit cost	26		33		52		65
Supplemental Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	13		8		25		15
Interest cost	40		40		79		80
Expected return on plan assets
Amortization of net actuarial loss	56	[1]	48	[1]	113	[1]	96	[1]
Net periodic benefit cost	$ 109		$ 96		$ 217		$ 191

[1] The total amount of amortization of net actuarial loss for all retirement plans was $622 and $523 for the three-month periods ended June 30, 2013, and 2012, respectively, and $1,245 and $1,050 for the six-month periods ended June 30, 2013, and 2012, respectively.