Note 4 - Long-term Debt (Details Textual) $ in Thousands	Mar. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	May 16, 2014 USD ($)
Credit Facility Syndication, Number of Commercial Banks			11
Line of Credit Facility, Maximum Borrowing Capacity Without Borrowing Base Requirement			$ 500,000
Line of Credit Facility, Accordion Feature, Additional Revolving Credit Commitments Under Existing Loan Facility			$ 100,000
Long-term Debt, Excluding Current Maturities	$ 275,843	$ 259,389
Letters of Credit Outstanding, Amount	3,600
Line of Credit Facility, Remaining Borrowing Capacity	220,600
Line of Credit Facility, Current Borrowing Capacity	$ 173,800
Minimum [Member]
Line of Credit Facility, Fixed Charge Coverage Ratio Covenant			1.25
Maximum [Member]
Line of Credit Facility, Leverage Ratio Covenant			3.75
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			$ 30,000