Accounts Receivable, Net and Supplemental Balance-Sheet Information (Details) - USD ($) $ in Thousands	Sep. 26, 2020	Dec. 31, 2019
Accounts receivable, net
Accounts receivable	$ 199,749	$ 176,849
Unbilled Receivables(1)	69,810	58,277
Accounts receivable, gross	269,559	235,126
Less allowances for doubtful accounts	3,871	3,815
Accounts receivable, net	265,688	231,311
Other current assets
Refundable income taxes	333	339
Prepaid expenses	33,123	25,664
Other	593	984
Total	34,049	26,987
Property, Plant and Equipment, Net [Abstract]
Land and Land Improvements	19,218	19,270
Buildings and leasehold improvements	46,693	44,414
Equipment	622,205	604,211
Property, Plant and Equipment, Gross	688,116	667,895
Less accumulated depreciation	482,822	468,045
Property and equipment, net	205,294	199,850
Other Assets, Noncurrent [Abstract]
Assets invested for self-insurance	15,359	15,426
Investment--cost-method affiliate	1,258	1,314
Other	4,681	5,595
Total	21,298	22,335
Accrued expenses
Employee compensation	29,642	26,381
Accrued compensated absences	10,831	10,744
Self-insured medical claims	4,296	1,824
Income tax payable	10,451	6,420
Customer advances, deposits	1,528	1,674
Taxes, other than income	20,990	1,775
Other	2,483	3,613
Total	80,221	52,431
Other current liabilities
Notes payable	338	1,853
Current operating lease liabilities	17,933	14,665
Current portion of long-term debt	33,065	23,302
Self-insurance reserve	30,246	30,882
Total	48,517	47,400
Other noncurrent liabilities
Pension and retirement plans	6,943	6,552
Deferred income taxes	649	567
Other	3,751	5,149
Other noncurrent liabilities	$ 11,343	$ 12,268