Employee Savings, Stock Purchase and Supplemental Executive Retirement Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Retirement Saving Opportunity Plan (RSOP)
Employee benefit plan
Eligibility requirement, initial period of employment for which part-time employees must complete 1,000 hours of service	12 months
Eligibility requirement, service hours that must be completed by part-time employees within the first 12 months of employment or within any plan year after the date of hire	1000 hours
Total compensation expense related to Company's matching contributions	$ 2,000,000	$ 1,800,000	$ 1,900,000
Employee Stock Purchase Plan (ESPP)
Employee benefit plan
Employer contribution as a percentage of employee's payroll deduction	20.00%
Total compensation expense related to Company's matching contributions	164,000	153,000	135,000
Supplemental Executive Retirement plan
Employee benefit plan
Retirement plan vesting period	5 years
Value of assets held in trust	13,100,000	11,100,000
Net periodic benefit (income) cost of plans	(599,000)	1,600,000	351,000
Death benefit	863,000
Service costs	121,000	347,000	204,000
Interest costs	1,200,000	731,000	660,000
Investment gain (loss)	1,100,000	(521,000)	513,000
Projected benefit obligations	$ 12,700,000	$ 11,300,000
Weighted-average discount rate (as a percent)	6.60%	7.00%	7.00%