Employee Benefit Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Profit Sharing Plan [Abstract]
Requisite service period	1 year
Number of annual employer contribution	2
Employer discretionary contribution amount	$ 800,000	$ 775,000	$ 725,000
Employer mandatory contributions amount	941,000	868,000	867,000
Annual vesting percentage, first year (in hundredths)	0.00%
Annual vesting percentage, full year thereafter (in hundredths)	25.00%
Vesting percentage after five years (in hundredths)	100.00%
Executive Retirement Plan and Life Insurance Arrangements [Abstract]
Vesting schedule for the Non-Qualified Executive Retirement Plan - Performance Component (in hundredths)	50.00%
Vesting schedule for the Non-Qualified Executive Retirement Plan - Salary Component	5 years
Vesting schedule for the Non-Qualified Executive Retirement Plan - Equity Component (in hundredths)	50.00%
Employer contributions to Non-Qualified Executive Retirement Plan	2,600,000	2,300,000	1,600,000
Accrued benefit liability under Non-Qualified Executive Retirement Plan	19,300,000	15,600,000
Tax-exempt interest on the life insurance policies	1,836,000	1,834,000	1,825,000
Cash surrender value of life insurance policies	50,300,000	47,400,000
Senior Management Retention Plan [Abstract]
Vesting period (in years)	5 years
Contribution to the Senior Management Retention Plan	$ 206,000