Long-Term Debt and Commitments Related to Letters of Credit Long-Term Debt (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Revolving credit facility, noncurrent	$ 128,500	$ 100,000
Senior unsecured notes	18,000	18,000
Term loans	11,651	16,242
Capital leases	2,087	2,510
Long-term debt	160,238	136,752
Less debt issuance costs	681	725
Less current portion	13,155	16,817
Long-term debt, excluding current maturities	146,402	119,210
Swing-line borrowings [Member]
Revolving credit facility, noncurrent	13,500	0
London Interbank Offered Rate (LIBOR) [Member]
Revolving credit facility, noncurrent	$ 115,000	$ 100,000