Components of Net Period Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Supplemental Executive Retirement Plan
Net period benefit cost
Service cost	$ 67	$ 58	$ 134	$ 116
Interest cost	102	108	204	216
Recognized net actuarial loss	29	17	58	34
Recognized prior service cost	5	4	10	8
Net period benefit cost	203	187	406	374
Other Postretirement Benefit Plan
Net period benefit cost
Service cost	17	16	34	32
Interest cost	37	38	74	76
Recognized net actuarial loss	8		16
Net period benefit cost	$ 62	$ 54	$ 124	$ 108