Unaudited Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 29, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Allowance for doubtful accounts	$ 1,602	$ 1,330
Property and equipment, Accumulated depreciation	227,146	205,771
Intangible assets, accumulated amortization	$ 60,113	$ 56,901
Common stock, par value
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	30,954,202	30,941,702
Common stock, shares outstanding	28,371,327	28,382,392
Treasury stock, shares	2,582,875	2,559,310
Income tax expense on unrealized holding gain on available-for-sale securities	$ 0	$ 1,090
Interest rate swaps, income taxes	$ 170	$ 63