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Long-Term Debt and Loan Agreements - Additional Information (Details) - USD ($)
3 Months Ended 9 Months Ended
May 30, 2014
Sep. 30, 2018
Sep. 30, 2017
Mar. 31, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Debt Instrument [Line Items]              
Long-term Debt   $ 78,000,000     $ 78,000,000   $ 152,632,000
Long-term Debt   $ 76,693,000     $ 76,693,000   $ 151,036,000
Weighted average interest rate during period   6.55% 5.18%   5.70% 5.05%  
Senior Unsecured Note              
Debt Instrument [Line Items]              
Long-term Debt   $ 78,000,000     $ 78,000,000    
Minimum [Member]              
Debt Instrument [Line Items]              
Long-term Debt   $ 11,000,000     $ 11,000,000    
Interest rate   4.67%     4.67%    
Maximum [Member]              
Debt Instrument [Line Items]              
Long-term Debt   $ 40,000,000     $ 40,000,000    
Interest rate   5.45%     5.45%    
Loan Agreement [Member]              
Debt Instrument [Line Items]              
Maximum borrowing capacity on line of credit $ 300,000,000     $ 200,000,000      
Loan maturity period 2018-12     2022-03      
Remaining amount available under the line of credit   $ 195,600,000     $ 195,600,000    
Letters of credit   $ 4,400,000     $ 4,400,000