Employee Retirement Plans (Details 3) (Pension benefits, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Employee retirement plans
Target (as a percent)	100.00%
Actual allocation of pension plan assets	$ 21,808	$ 18,637	$ 17,804
Weighted-average asset allocation (as a percent)	100.00%	100.00%
Large Cap Equity
Employee retirement plans
Target (as a percent)	37.00%
Actual allocation of pension plan assets	6,766	5,901
Weighted-average asset allocation (as a percent)	31.00%	32.00%
Mid Cap Equity
Employee retirement plans
Target (as a percent)	4.00%
Actual allocation of pension plan assets	859	750
Weighted-average asset allocation (as a percent)	4.00%	4.00%
Small Cap Equity
Employee retirement plans
Target (as a percent)	3.00%
Actual allocation of pension plan assets	876	783
Weighted-average asset allocation (as a percent)	4.00%	4.00%
International Equity
Employee retirement plans
Target (as a percent)	12.00%
Actual allocation of pension plan assets	2,448	2,041
Weighted-average asset allocation (as a percent)	11.00%	11.00%
Emerging markets Equity
Employee retirement plans
Target (as a percent)	2.00%
Actual allocation of pension plan assets	598	353
Weighted-average asset allocation (as a percent)	3.00%	2.00%
Fixed Income and Cash Equivalents
Employee retirement plans
Target (as a percent)	34.00%
Actual allocation of pension plan assets	8,019	6,966
Weighted-average asset allocation (as a percent)	37.00%	37.00%
Real Estate
Employee retirement plans
Target (as a percent)	8.00%
Actual allocation of pension plan assets	$ 2,242	$ 1,843
Weighted-average asset allocation (as a percent)	10.00%	10.00%