Long-Term Debt and Commitments Related to Letters of Credit (Narrative) (Details) $ in Thousands	9 Months Ended
	Sep. 26, 2020 USD ($)	May 18, 2020	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Revolving credit facility, current borrowing capacity	$ 250,000
Revolving credit facility, maximum borrowing capacity	325,000
Unused commitments under credit facility	217,123
Revolving credit facility borrowings	$ 30,000		$ 62,000
Number of principal payments	5
Long-term debt, maturities, repayments of principal in next twelve months	$ 8,025
Long-term debt, maturities, repayments of principal in year two	25,768
Long-term debt, maturities, repayments of principal in year three	30,942
Long-term debt, maturities, repayments of principal in year four	172
Long-term debt, maturities, repayments of principal in year five	15,000
Long-term debt, maturities, repayments of principal after year five	$ 60,000
Anniversary when principal payments begin	6
Minimum [Member]
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage	0.10%
Maximum [Member]
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage	0.225%
Swing-line borrowings [Member]
Debt Instrument [Line Items]
Revolving credit facility, current borrowing capacity	$ 25,000
Revolving credit facility borrowings	$ 18,000		10,000
Revolving credit facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest coverage ratio	3.00
Revolving credit facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Ratio of Indebtedness to Net Capital	3.00
5.09% Senior unsecured notes [Member]
Debt Instrument [Line Items]
Interest rate	5.09%
Debt Instrument, Face Amount	$ 30,000
3.99% Senior unsecured notes [Member]
Debt Instrument [Line Items]
Interest rate	3.99%
Unsecured debt maximum borrowing capacity allowed by credit facility	$ 75,000
Unsecured debt additional shelf note capacity	50,000
Debt Instrument, Face Amount	50,000
Unsecured debt additional shelf note capacity allowed by credit facility	$ 25,000
4.00% Senior unsecured notes [Member]
Debt Instrument [Line Items]
Interest rate	4.00%
Unsecured debt additional shelf note capacity	$ 25,000
Accounts receivable securitization [Member]
Debt Instrument [Line Items]
Interest rate	1.00%	0.90%
Unused commitment fee threshold percent	0.50
Debt instrument variable rate base rate calculation, default rate	2.00%
Accounts receivable securitization [Member] | Base Rate, Federal Funds [Member]
Debt Instrument [Line Items]
Outstanding borrowings interest rate margin adjustment, range	0.50%
Line of credit [Member]
Debt Instrument [Line Items]
Revolving credit facility borrowings	$ 32,877
Line of credit [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Outstanding borrowings interest rate margin adjustment, range	1.50%
Line of credit [Member] | Base Rate, Federal Funds [Member]
Debt Instrument [Line Items]
Outstanding borrowings interest rate margin adjustment, range	0.50%
Line of credit [Member] | Minimum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Outstanding borrowings interest rate margin adjustment, range	0.875%
Line of credit [Member] | Maximum [Member] | London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument [Line Items]
Outstanding borrowings interest rate margin adjustment, range	1.50%
Letter of credit [Member]
Debt Instrument [Line Items]
Letters of credit issued	$ 88,242		81,619
Letter of credit [Member] | Revolving credit facility [Member]
Debt Instrument [Line Items]
Letters of credit issued	2,877		2,877
Letter of credit [Member] | Accounts receivable securitization [Member]
Debt Instrument [Line Items]
Revolving credit facility, maximum borrowing capacity	100,000
Letters of credit issued	$ 83,355		76,732
Debt instrument, term	1 year
Letter of credit [Member] | Accounts receivable securitization [Member] | Minimum [Member]
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage	0.35%
Letter of credit [Member] | Accounts receivable securitization [Member] | Maximum [Member]
Debt Instrument [Line Items]
Line of credit facility, unused capacity, commitment fee percentage	0.45%
Letter of credit [Member] | Line of credit [Member]
Debt Instrument [Line Items]
Letters of credit issued	$ 2,010		$ 2,010
Letter of credit [Member]
Debt Instrument [Line Items]
Revolving credit facility, maximum borrowing capacity	$ 100,000