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		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Defined-Benefit Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 216	$ 193	$ 647	$ 580
Interest cost	477	454	1,430	1,364
Expected return on plan assets	(509)	(512)	(1,527)	(1,538)
Amortization of net loss	471	343	1,414	1,028
Net periodic benefit cost	655	478	1,964	1,434
Postretirement Healthcare Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	10	9	28	28
Interest cost	18	20	54	61
Amortization of net loss	5	7	16	21
Net periodic benefit cost	33	36	98	110
Supplemental Retirement Plan
Defined Benefit Plan Disclosure [Line Items]
Service cost	7	5	22	15
Interest cost	40	40	120	120
Amortization of net loss	48	38	144	114
Net periodic benefit cost	$ 95	$ 83	$ 286	$ 249