Employee Benefits Plans (Detail) - Components of benefit cost (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental Employee Retirement Plans, Defined Benefit [Member]
Interest cost	$ 82	$ 93	$ 101
Net amortization and deferral	223	179	94
Net periodic benefit cost (income)	305	272	195
Other Postretirement Benefit Plans, Defined Benefit [Member]
Interest cost	49	61	82
Amortization of transition obligation	(177)	(177)	(153)
Amortization of net actuarial loss	11	8	47
Net periodic benefit cost (income)	$ (117)	$ (108)	$ (24)