Debt and Credit Arrangements - Effect of Derivatives on Financial Statements (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 25, 2016	Dec. 27, 2015
Debt and Credit Arrangements
Total Interest Expense on Consolidated Statements of Income	$ (11,283,000)	$ (7,397,000)	$ (5,676,000)
Weighted average interest rates on debt, including the impact of interest rate swap agreements	2.70%	2.10%	2.00%
Interest paid, including payments made or received under the swaps	$ 10,800,000	$ 7,100,000	$ 5,300,000
Interest rate swap | Other current and long-term assets
Debt and Credit Arrangements
Interest rate swap assets	$ 651,000
Estimate of period of time over which portion of derivative liability would be reclassified into earnings	12 months
Portion of derivative asset that would be reclassified into earnings	$ 281,000
Interest expense | Interest rate swap
Debt and Credit Arrangements
Amount of Gain or (Loss) Recognized in AOCL on Derivative	891,000	940,000	(1,163,000)
Amount of Gain or (Loss) Reclassified from AOCL into Income	(421,000)	(1,161,000)	(1,563,000)
Total Interest Expense on Consolidated Statements of Income	$ (11,283,000)	$ (7,397,000)	$ (5,676,000)