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Leases - Future Minimum Lease Payments (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Leases [Abstract]      
Rent expense $ 4,200 $ 4,000 $ 3,600
Future minimum non-cancelable operating lease payments      
2018 4,411    
2019 3,333    
2020 2,347    
2021 2,204    
2022 1,649    
Thereafter 443    
Total operating lease commitments 14,387    
Operating Leased Assets [Line Items]      
Liability related to rent escalations 1,200 $ 1,100  
Allowance for moving expenses and tenant improvements $ 1,500    
Property subject to rent escalations | Minimum      
Operating Leased Assets [Line Items]      
Term of lease 48 months    
Property subject to rent escalations | Maximum      
Operating Leased Assets [Line Items]      
Term of lease 125 months    
Corporate headquarters      
Operating Leased Assets [Line Items]      
Term of lease 125 months