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Fair Value Measurement - Carrying and Fair Value Amounts (Details) - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities $ 132,790  
Credit Agreement Loan, carrying value 290,000  
Reported Value Measurement [Member] | Held-to-Maturity [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 132,790  
Reported Value Measurement [Member] | Level 1 [Member] | Held-to-Maturity [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities   $ 127,779
Held-to-maturity marketable securities, carrying value 132,790 127,779
Reported Value Measurement [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long term portion of term loan 90,000 95,000
Reported Value Measurement [Member] | Level 3 [Member] | Senior Notes [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Senior notes payable, carrying value 80,000 120,000
Estimate of Fair Value Measurement [Member] | Level 1 [Member] | Held-to-Maturity [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity Securities 132,002 127,365
Estimate of Fair Value Measurement [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long term portion of term loan [1] 89,871 93,991
Estimate of Fair Value Measurement [Member] | Level 3 [Member] | Senior Notes [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Senior notes payable, carrying value [1] 82,190 124,654
Revolving Credit Facility [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Credit Agreement Loan, carrying value 200,000  
Long-term Line of Credit, Noncurrent 55,000 30,000
2017 Draw [Member] | Estimate of Fair Value Measurement [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Line of Credit, Noncurrent [1] 24,949 0
2017 Draw [Member] | Revolving Credit Facility [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Line of Credit, Noncurrent 25,000  
2017 Draw [Member] | Revolving Credit Facility [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Line of Credit, Noncurrent   0
2017 Draw [Member] | Revolving Credit Facility [Member] | Reported Value Measurement [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Line of Credit, Noncurrent 25,000  
2016 Draw [Member] | Estimate of Fair Value Measurement [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Line of Credit, Noncurrent [1] 30,105 29,452
2016 Draw [Member] | Revolving Credit Facility [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Line of Credit, Noncurrent   $ 30,000
2016 Draw [Member] | Revolving Credit Facility [Member] | Reported Value Measurement [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Long-term Line of Credit, Noncurrent $ 30,000  
[1] 1The fair values of the senior notes payable and Credit Agreement (defined in Note 11) loan are based on borrowing rates available to us for long-term loans with similar terms, average maturities, and credit risk.