Quarterly Results of Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 29, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jul. 01, 2017	Apr. 01, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 280,173	$ 265,318	$ 270,649	$ 208,651	$ 228,520	$ 249,588	$ 245,037	$ 192,813	$ 1,024,791	$ 915,958	$ 845,678
Gross profit (revenues less costs and expenses, operating)	98,215	94,193	102,967	64,028	83,986	91,973	93,512	59,154
Income (loss) from operations	16,971	14,585	29,800	(5,666)	11,478	19,049	25,659	(3,995)	55,690	52,191	48,867
Net income (loss)	$ 4,506	$ 8,384	$ 21,714	$ (6,627)	$ 1,547	$ 10,237	$ 14,223	$ (3,887)	$ 27,977	$ 22,120	$ 22,284
Net income (loss) per share -- Basic	$ 0.12	$ 0.35	$ 0.94	$ (0.26)	$ 0.04	$ 0.41	$ 0.57	$ (0.15)	$ 1.15	$ 0.87	$ 0.85
Net income (loss) per share -- Diluted	0.13	0.34	0.89	(0.26)	0.04	0.39	0.55	(0.15)	$ 1.10	$ 0.83	$ 0.82
ESOT valuation per share	$ 21.10	$ 19.70	$ 19.70	$ 19.10	$ 19.10	$ 18.30	$ 18.30	$ 17.60
Decrease in casualty insurance expense	$ (7,324)				$ 2,735