Employee Retirement Benefits (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Employee Retirement Benefits [Abstract]
Employer contribution in retirement savings plan (in hundredths)	6.00%
Employer contribution in retirement savings plan	$ 1,900,000	$ 1,581,000	$ 1,538,000
Expected post-retirement benefits expected to be paid for next 5 years [Abstract]
2013	288,000
2014	398,000
2015	435,000
2016	477,000
2017	477,000
Thereafter	3,258,000
Change in benefit obligation [Abstract]
Benefit obligation at beginning of year	17,326,000	14,971,000
Service cost	599,000	509,000	452,000
Interest cost	797,000	799,000	723,000
Amendments	(7,306,000)	(216,000)
Actuarial loss	(918,000)	1,632,000
Benefits paid	(488,000)	(369,000)
Valuation adjustment	1,034,000	0	0
Benefit obligation at end of year	11,044,000	17,326,000	14,971,000
Components of net periodic benefit cost
Service cost	599,000	509,000	452,000
Interest cost	797,000	799,000	723,000
Expected return on plan assets	0	0	0
Amortization of transition obligation	60,000	60,000	60,000
Amortization of prior service cost	(76,000)	(55,000)	(55,000)
Recognized net actuarial loss	630,000	429,000	391,000
Net periodic benefit cost	2,010,000	1,742,000	1,571,000
Discount rate used to determine benefit obligation	4.40%	4.40%	5.40%
Transition obligation	0
Prior service credit	(945,000)
Net actuarial loss	479,000
Effect of 1% Increase and 1% Decrease in Trend Rate [Abstract]
Effect of 1% increase on accumulated post-retirement benefit obligation	294,000
Percentage change in retiree medical costs, Effect of 1% increase on accumulated post-retirement benefit obligation	2.85%
Effect of 1% decrease on accumulated post-retirement benefit obligation	(256,000)
Percentage change in retiree medical costs, Effect of 1% decrease on accumulated post-retirement benefit obligation	(2.48%)
Effect of 1% increase on service and interest cost for fiscal 2011	286,000
Effect of 1% decrease on service and interest cost for fiscal 2011	(223,000)
Reduction in accumulated post-retirement benefit obligation	$ 49,000	$ 2,833,000
Pre-Medicare [Member]
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]
2013 (in hundredths)	8.50%
2014 (in hundredths)	8.00%
2015 (in hundredths)	7.50%
2016 (in hundredths)	7.00%
2017 (in hundredths)	6.50%
2018 (in hundredths)	6.00%
2019 (in hundredths)	5.50%
2020 (in hundredths)	5.00%
Ultimate (in hundredths)	4.50%
Year that rate reaches ultimate trend rate	2021
Post-Medicare [Member]
Defined Benefit Plan, Assumed Health Care Cost Trend Rates [Abstract]
2013 (in hundredths)	7.00%
2014 (in hundredths)	6.90%
2015 (in hundredths)	6.80%
2016 (in hundredths)	6.70%
2017 (in hundredths)	6.60%
2018 (in hundredths)	6.50%
2019 (in hundredths)	6.40%
2020 (in hundredths)	6.30%
Ultimate (in hundredths)	4.50%
Year that rate reaches ultimate trend rate	2038