Pension and Profit Sharing Plans - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 Y	Dec. 31, 2011	Dec. 31, 2010
Pension And Profit Sharing Plans [Line Items]
Contributions towards pension plan	$ 2,000,000	$ 1,500,000
Weighted average final maturities of debt securities held in the pension plan, Years	4.24
Level 3 securities	0
Common stock value	315,000	314,000
Vesting period	6 years
Defined contribution plan cost	4,711,000	4,688,000	4,299,000
Employee deferrals	4.00%
Employee compensation	5.00%
Directors' fees	100.00%
Pension plan's assets [Member]
Pension And Profit Sharing Plans [Line Items]
Common stock value	1,202,000	1,030,000
Profit sharing plan's assets [Member]
Pension And Profit Sharing Plans [Line Items]
Common stock value	29,043,000	25,715,000
Safe harbor 401(k) plan [Member]
Pension And Profit Sharing Plans [Line Items]
Total expense	$ 1,383,000	$ 1,305,000	$ 1,220,000
Minimum [Member]
Pension And Profit Sharing Plans [Line Items]
Weighted average final maturities of debt securities held in the pension plan, Years	1.75
Maximum [Member]
Pension And Profit Sharing Plans [Line Items]
Weighted average final maturities of debt securities held in the pension plan, Years	8.67