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Fair Value Measurement - Carrying and Fair Value Amounts (Details) - USD ($)
$ in Thousands
Dec. 31, 2015
Dec. 31, 2014
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 marketable securities, fair value $ 105,336 $ 101,808
Estimate of Fair Value Measurement [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Credit Agreement Loan, fair value [1] 99,375 70,153
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, fair value [1] 165,731 220,226
Reported Value Measurement [Member] | Level 1 [Member] | Held-to-Maturity [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Held-to-maturity marketable securities, carrying value 105,695 102,067
Reported Value Measurement [Member] | Level 3 [Member]    
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]    
Credit Agreement Loan, carrying value 100,000 70,000
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 $ 160,000 $ 200,000
[1] The fair values of the senior notes payable and Credit Agreement (defined in Note 12) loan are based on borrowing rates available to us for long-term loans with similar terms, average maturities, and credit risk.