Components of Net Period Benefit Cost (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
Supplemental Executive Retirement Plan
Net period benefit cost
Service cost	$ 67	$ 58	$ 201	$ 174
Interest cost	102	108	306	324
Recognized net actuarial loss	29	17	87	51
Recognized prior service cost	5	4	15	12
Net period benefit cost	203	187	609	561
Other Postretirement Benefit Plan
Net period benefit cost
Service cost	17	16	51	48
Interest cost	37	38	111	114
Recognized net actuarial loss	8		24
Net period benefit cost	$ 62	$ 54	$ 186	$ 162