Retirement Plans - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Discount rate on benefit obligation	3.87%	4.41%
Increase in the fair value of plan assets	$ 4,300,000
Increase in projected benefit obligation	5,600,000
Increase in projected benefit obligation on plan assets	17,100,000	15,800,000
Expected return on plan assets	7.00%
Target allocation of percentage, equity securities	60.00%
Target allocation of percentage, debt securities	40.00%
Settlement charge included in net periodic cost for retirement plans	0	0	1,200
Age limit of retired employees to receive company subsidy	64
Postretirement benefit liability	1,800,000	1,900,000
Postretirement benefit expense	100,000	100,000	200,000
Assumed health care cost trend rate used in measuring the post retirement benefit obligation	8.50%
Ultimate rate of health care cost trend	5.00%
Percent of change in health care trend rate	1.00%
Change in health care trend rate would change benefit obligation	200,000
Change in health care trend rate would change aggregate service and interest cost	100,000
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate on benefit obligation	3.87%	4.41%
Defined benefit cost excluding settlement charges	2,618,000	1,912,000	1,548,000
Expected return on plan assets	7.00%	8.00%
Estimated for future contributions	2,000,000
Accumulated benefit obligation	45,700,000	40,300,000
Benefit Equalization Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit cost excluding settlement charges	381,000	332,000	356,000
Estimated for future contributions	200,000
Accumulated benefit obligation	3,700,000	3,300,000
Prepaid pension cost	100,000	300,000
Postretirement Plan
Defined Benefit Plan Disclosure [Line Items]
Discount rate on benefit obligation	3.26%	3.91%
Estimated for future contributions	100,000
Pre-Age Retirees
Defined Benefit Plan Disclosure [Line Items]
Age eligibility for retired employees to participate in health care plans	65
Grandfathered Post-Age Retirees
Defined Benefit Plan Disclosure [Line Items]
Age eligibility for retired employees to participate in health care plans	65
401(k) Plan
Defined Benefit Plan Disclosure [Line Items]
Matching contribution contributed by the company under the terms of the plan	50.00%
Percent of maximum contribution by the employer for eligible employee compensation for participant employee under the plan	3.00%
Percent of maximum contribution by the employer for eligible employee compensation for non participant employee under the plan	6.00%
Percent of maximum additional discretionary matching contribution by the employer up to the maximum eligible employee compensation	30.00%
Defined contribution	$ 2,800,000	$ 2,700,000	$ 2,100,000