Fair Value Measurements and Financial Instruments (Details) - Fair value, measurements, recurring [Member] - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Quoted prices in active markets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets invested for self-insurance, classified as other assets, noncurrent	$ 14,909	$ 19,422
Deferred compensation	0	0
Significant other observable inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets invested for self-insurance, classified as other assets, noncurrent	0	0
Deferred compensation	2,310	2,146
Significant unobservable inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets invested for self-insurance, classified as other assets, noncurrent	0	0
Deferred compensation	0	0
Total carrying value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets invested for self-insurance, classified as other assets, noncurrent	14,909	19,422
Deferred compensation	$ 2,310	$ 2,146