Quarterly Results of Operations (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 29, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Oct. 01, 2011	Jul. 02, 2011	Apr. 02, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 172,451	$ 178,647	$ 182,411	$ 146,644	$ 161,958	$ 178,799	$ 173,953	$ 131,324	$ 680,153	$ 646,034	$ 591,732
Gross profit (revenues less costs and expenses, operating)	65,577	64,544	65,939	46,760	58,561	62,642	62,711	35,494
Income (loss) from operations	13,337	14,652	17,270	508	11,112	13,342	16,567	(8,220)	45,767	32,801	29,612
Net income (loss)	2,564	13,418	9,416	(803)	3,990	6,962	9,159	(6,046)	24,595	14,065	14,053
Net income (loss) per share--Basic	$ 0.51	$ 0.59	$ 0.68	$ (0.04)	$ 0.28	$ 0.50	$ 0.65	$ (0.43)	$ 1.74	$ 1.00	$ 0.97
Net income (loss) per share -- Diluted:	$ 0.50	$ 0.56	$ 0.66	$ (0.04)	$ 0.28	$ 0.49	$ 0.63	$ (0.43)	$ 1.68	$ 0.97	$ 0.93
ESOT Valuation per share	$ 23.20	$ 20.80	$ 19.70	$ 19.70	$ 19.70	$ 18.00	$ 18.40	$ 18.40
Decrease in casualty insurance expense	5,136				5,737
Litigation settlement					2,900				0	2,900	0
Other-than-temporary impairment loss					$ 366					$ 366