Quarterly Results of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 28, 2013	Jun. 29, 2013	Mar. 30, 2013	Dec. 31, 2012	Sep. 29, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 180,173	$ 194,772	$ 195,638	$ 143,265	$ 172,451	$ 178,647	$ 182,411	$ 146,644	$ 713,848	$ 680,153	$ 646,034
Gross profit (revenues less costs and expenses, operating)	63,837	69,114	76,046	42,205	65,578	64,544	65,939	46,760
Income (loss) from operations	8,687	14,881	23,230	(6,009)	13,337	14,652	17,270	508	40,789	45,767	32,801
Net income (loss)	5,385	8,644	13,436	(4,612)	7,454	8,296	9,416	(571)	22,853	24,595	14,065
Net income (loss) per share--Basic	$ 0.34	$ 0.63	$ 0.97	$ (0.33)	$ 0.51	$ 0.59	$ 0.68	$ (0.04)	$ 1.61	$ 1.74	$ 1.00
Net income (loss) per share -- Diluted:	$ 0.35	$ 0.61	$ 0.94	$ (0.33)	$ 0.50	$ 0.56	$ 0.66	$ (0.04)	$ 1.57	$ 1.68	$ 0.97
ESOT Valuation per share	$ 26.40	$ 23.90	$ 23.20	$ 23.20	$ 23.20	$ 20.80	$ 19.70	$ 19.70
Decrease in casualty insurance expense	$ 5,402				$ 5,136