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Commitments and Contingencies - Rental Expense and Letters of Credit (Details)
12 Months Ended
Dec. 31, 2018
USD ($)
item
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Other Commitments [Line Items]      
Rental expense under operating leases $ 1,604,000 $ 1,785,000 $ 1,907,000
Obligations related to health insurance and self-insured workers' compensation      
Number of letters of credit issued | item 3    
Insurance Obligations      
Obligations related to health insurance and self-insured workers' compensation      
Letters of credit issued $ 1,767,000    
Workers' Compensation Insurance      
Obligations related to health insurance and self-insured workers' compensation      
Letters of credit issued 583,000    
Performance Obligations      
Obligations related to health insurance and self-insured workers' compensation      
Letters of credit issued 75,000    
Letter Of Credit      
Obligations related to health insurance and self-insured workers' compensation      
Funds borrowed under Line of Credit $ 0