Earnings Per Share Information (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
Earnings Per Share [Abstract]
Net income	$ 4,506	$ 8,384	$ 21,714	$ (6,627)	$ 1,547	$ 10,237	$ 14,223	$ (3,887)	$ 27,977	$ 22,120	$ 22,284
Weighted-average shares, basic
Outstanding									23,698,478	24,793,086	25,378,600
Partially-paid share subscriptions									690,260	764,382	833,440
Basic weighted-average shares									24,388,738	25,557,468	26,212,040
Weighted-average shares, diluted
Exercise of stock subscription purchase rights									141,287	146,453	142,746
Exercise of stock options and awards									950,593	993,141	892,568
Diluted weighted-average shares									25,480,618	26,697,062	27,247,354
Earnings 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
Earnings 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