Accounts Receivable, Net and Supplemental Balance-Sheet Information (Details) - USD ($) $ in Thousands	Jun. 27, 2020	Dec. 31, 2019
Accounts receivable, net
Accounts receivable	$ 204,207	$ 176,849
Unbilled Receivables(1)	68,509	58,277
Accounts receivable, gross	272,716	235,126
Less allowances for doubtful accounts	3,796	3,815
Accounts receivable, net	268,920	231,311
Other current assets
Refundable income taxes	0	339
Prepaid expenses	11,258	25,664
Other	712	984
Total	11,970	26,987
Property, Plant and Equipment, Net [Abstract]
Land and Land Improvements	19,169
Buildings and leasehold improvements	45,293
Equipment	617,446
Property, Plant and Equipment, Gross	681,908
Less accumulated depreciation	474,652
Property and equipment, net	207,256	199,850
Other Assets, Noncurrent [Abstract]
Assets invested for self-insurance	12,000	15,426
Investment--cost-method affiliate	1,267	1,314
Other	4,386	5,595
Total	17,653	22,335
Accrued expenses
Employee compensation	20,298	26,381
Accrued compensated absences	10,727	10,744
Self-insured medical claims	4,104	1,824
Income tax payable	14,809	6,420
Customer advances, deposits	1,367	1,674
Taxes, other than income	11,043	1,775
Other	3,513	3,613
Total	65,861	52,431
Other current liabilities
Notes payable	0	1,853
Current operating lease liabilities	17,463	14,665
Current portion of long-term debt	16,495	23,302
Self-insurance reserve	30,632	30,882
Total	48,095	47,400
Other noncurrent liabilities
Pension and retirement plans	6,830	6,552
Deferred income taxes	503	567
Other	4,038	5,149
Other noncurrent liabilities	$ 11,371	$ 12,268