Note 3 - Long-term Debt (Details Textual) $ in Thousands	May 17, 2017 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Credit Facility Syndication, Number of Commercial Banks	11
Line of Credit Facility, Maximum Borrowing Capacity Without Borrowing Base Requirement	$ 600,000
Line of Credit Facility, Accordion Feature, Additional Revolving Credit Commitments Under Existing Loan Facility	300,000
New Syndicated Borrowings Used to Pay Off or Pay Down Borrowings from Syndicate Members	$ 106,000
Line of Credit Facility, Interest Coverage Ratio Covenant	3
Line of Credit Facility, Leverage Ratio Covenant	3.75
Line of Credit Facility, Adjusted Leverage Ratio Covenant Under Certain Circumstances	4
Long-term Debt, Excluding Current Maturities		$ 278,000	$ 259,389
Letters of Credit Outstanding, Amount		3,400
Line of Credit Facility, Remaining Borrowing Capacity		318,600
Line of Credit Facility, Current Borrowing Capacity		$ 184,500
Weighted Average [Member]
Long-term Debt, Weighted Average Interest Rate, at Point in Time		2.60%
Line of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 60,000
Federal Funds Effective Swap Rate [Member]
Debt Instrument, Basis Spread on Variable Rate	0.50%
London Interbank Offered Rate (LIBOR) [Member]
Debt Instrument, Basis Spread on Variable Rate	1.00%