Note 7 - Long-term Debt (Details Textual) (USD $)	0 Months Ended	3 Months Ended
	May 16, 2014	Mar. 31, 2015	Dec. 31, 2014
Credit Facility Syndication, Number of Commercial Banks	11
Line of Credit Facility, Expiration Date	May 16, 2019
Line of Credit Facility, Maximum Borrowing Capacity Without Borrowing Base Requirement	$ 500,000,000
Line of Credit Facility, Accordion Feature, Additional Revolving Credit Commitments Under Existing Loan Facility	100,000,000
Long-term Debt, Excluding Current Maturities		377,299,000	350,052,000
Letters of Credit Outstanding, Amount		6,200,000
Line of Credit Facility, Remaining Borrowing Capacity		116,500,000
Line of Credit Facility, Current Borrowing Capacity		80,200,000
Letter of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity	$ 30,000,000
Minimum [Member]
Line of Credit Facility, Fixed Charge Coverage Ratio Covenant		1.25
Line of Credit Facility, Interest Rate During Period		2.17%
Maximum [Member]
Line of Credit Facility, Leverage Ratio Covenant		3.75
Line of Credit Facility, Interest Rate During Period		4.25%