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Note 6 - Long-Term Debt (Details Textual) - USD ($)
$ in Thousands
Jun. 30, 2019
Dec. 31, 2018
Long-term Line of Credit, Total $ 1,696,282 $ 1,771,330
Revolving Credit Facility [Member]    
Line of Credit Facility, Maximum Borrowing Capacity 945,500  
Line of Credit [Member]    
Long-term Line of Credit, Total 1,700  
Line of Credit Facility, Remaining Borrowing Capacity 574,200  
Line of Credit [Member] | Swing Loan [Member]    
Long-term Line of Credit, Total 38,600 50,300
Line of Credit [Member] | Revolving Credit Facility [Member]    
Long-term Line of Credit, Total 320,000 $ 320,000
Line of Credit [Member] | Letter of Credit [Member]    
Long-term Line of Credit, Total $ 12,700