Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Oct. 01, 2016	Jul. 02, 2016	Apr. 02, 2016	Dec. 31, 2015	Oct. 03, 2015	Jul. 04, 2015	Apr. 04, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 216,363	$ 223,719	$ 224,763	$ 180,833	$ 197,858	$ 224,981	$ 224,773	$ 174,292	$ 845,678	$ 821,904	$ 789,911
Gross profit (revenues less costs and expenses, operating)	77,515	81,386	87,631	57,660	69,331	81,808	87,432	54,434
Income (loss) from operations	11,163	15,042	24,286	(5,120)	4,120	16,177	27,796	(3,985)	45,371	44,108	44,003
Net income (loss)	$ 5,022	$ 8,141	$ 13,309	$ (4,188)	$ 1,220	$ 8,174	$ 15,932	$ (3,528)	$ 22,284	$ 21,798	$ 23,169
Net income (loss) per share -- Basic	$ 0.37	$ 0.63	$ 1.03	$ (0.33)	$ 0.10	$ 0.58	$ 1.21	$ (0.27)	$ 1.70	$ 1.62	$ 1.68
Net income (loss) per share -- Diluted	0.36	0.61	1.00	(0.33)	0.11	0.55	1.17	(0.27)	$ 1.64	$ 1.56	$ 1.63
ESOT valuation per share	$ 35.20	$ 33.90	$ 32.70	$ 32.70	$ 32.70	$ 31.50	$ 30.10	$ 30.10
Decrease in casualty insurance expense	$ 2,810				$ 2,589