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Long-term Debt and Line of Credit - Provisions of the Term Loan (Details) - USD ($)
3 Months Ended 12 Months Ended
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Aug. 05, 2015
Debt Instrument [Line Items]          
Quarterly principal payments   $ 11,800,000      
Repayments of Debt   87,813,000 $ 63,084,000 $ 42,955,000  
Current portion of debt $ 13,500,000 13,500,000      
Non-current portion of debt 65,300,000 65,300,000      
Term Loan | Line of Credit          
Debt Instrument [Line Items]          
Line of credit facility, amount outstanding 78,800,000 78,800,000      
Repayments of Debt   6,000,000      
Line of credit facility, maximum borrowing capacity 135,000,000 135,000,000     $ 135,000,000
Revolving Credit Facility          
Debt Instrument [Line Items]          
Line of credit facility, amount outstanding 10,000,000 $ 10,000,000      
Repayments of Debt $ 70,000,000        
Revolving Credit Facility | Line of Credit          
Debt Instrument [Line Items]          
Line of credit facility, maximum borrowing capacity         $ 50,000,000
LIBOR | Term Loan | Line of Credit          
Debt Instrument [Line Items]          
Effective interest rate 5.13% 5.13%