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Note 9 - Long-term Debt (Details Textual)
$ in Thousands
12 Months Ended
Dec. 31, 2018
USD ($)
Dec. 31, 2017
USD ($)
Senior Notes, Total $ 150,000  
Number of Annual Principal Payments 5  
Facility and Senior Notes [Member]    
Debt Instrument, Interest Rate, Effective Percentage 3.80%  
Revolving Credit Facility [Member]    
Line of Credit Facility, Maximum Borrowing Capacity $ 250,000  
Debt Instrument, Term 5 years  
Debt, Weighted Average Interest Rate 3.75%  
Line of Credit Facility, Remaining Borrowing Capacity $ 67,540  
Letters of Credit Outstanding, Amount 5,214 $ 5,389
Line of Credit Facility Additional Borrowing Capacity $ 100,000  
Revolving Credit Facility [Member] | Minimum [Member]    
Debt Instrument, Basis Spread on Variable Rate 0.25%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.25%  
Revolving Credit Facility [Member] | Maximum [Member]    
Debt Instrument, Basis Spread on Variable Rate 2.50%  
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.50%  
Senior Notes [Member]    
Debt Instrument, Interest Rate, Stated Percentage 3.84%