Employee Savings, Stock Purchase, Long-Term Incentive and Supplemental Executive Retirement Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Employee benefit plan
Amortization of actuarial loss on defined benefit pension plan, gross	$ 28,000
Retirement Saving Opportunity Plan (RSOP)
Employee benefit plan
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 (in days)	125 days
Eligibility requirement, initial period of employment for which part-time employees must complete 1,000 hours of service	12 months
Total compensation expense related to Company's matching contributions	2,700,000	2,200,000	2,000,000
Employee Stock Purchase Plan (ESPP)
Employee benefit plan
Percentage employer matches of the employee's percentage contribution matched	20.00%
Total compensation expense related to Company's matching contributions	246,000	193,000	164,000
Supplemental Executive Retirement plan
Employee benefit plan
Retirement plan vesting period	5 years
Value of assets held in trust	15,700,000	15,200,000
Service costs		41,000	121,000
Interest costs	733,000	1,000,000	1,200,000
Amortization of actuarial loss on defined benefit pension plan, gross	28,000
Total costs	761,000	1,041,000	1,321,000
Benefits paid	90,000		863,000
Actuarial loss on the projected benefit obligation due a to a change in discount rate		1,700,000
Investment gain (loss)	657,000	2,100,000	1,100,000
Projected benefit obligations	16,100,000	15,400,000
Weighted-average discount rate (as a percent)	2.20%	2.00%	5.50%
Expected future payouts
2015	1,500,000
2016	90,000
2017	5,700,000
2018	4,600,000
2019	1,500,000
2020 through 2024	$ 3,200,000