Short and Long-Term Debt and Commitments Related to Letters of Credit Long-Term Debt (Details) - USD ($) $ in Thousands	Feb. 05, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt instrument [Line Items]
Notes payable		$ 0	$ 149
Short-term debt		22,514	16,315
Revolving credit facility, noncurrent		93,500	100,000
Term loans		23,176	16,242
Long-term debt		178,676	134,242
Less debt issuance costs		599	725
Less current portion		22,514	16,166
Long-term debt, excluding current maturities		155,563	117,351
Swing-line borrowings [Member]
Debt instrument [Line Items]
Revolving credit facility, noncurrent		2,500	0
LIBOR borrowings [Member]
Debt instrument [Line Items]
Revolving credit facility, noncurrent		91,000	100,000
5.09% Senior unsecured notes [Member]
Debt instrument [Line Items]
Senior unsecured notes		12,000	18,000
3.99% Senior unsecured notes [Member]
Debt instrument [Line Items]
Senior unsecured notes		50,000	0
Letter of credit [Member]
Debt instrument [Line Items]
Letters of credit outstanding		72,565	63,242
Letter of credit [Member] | Accounts receivable securitization [Member]
Debt instrument [Line Items]
Letters of credit outstanding		$ 67,438	$ 58,150
Subsequent event [Member] | 3.99% Senior unsecured notes [Member]
Debt instrument [Line Items]
Unsecured debt additional shelf note capacity allowed by credit facility	$ 25,000