Note I - Long-term Debt (Details Textual)	12 Months Ended
	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Nov. 05, 2014 USD ($)	May. 16, 2014
Amended Agreement [Member]
Line of Credit Facility, Expiration Date	May 16, 2019
Line of Credit Facility, Maximum Borrowing Capacity Without Borrowing Base Requirement				$ 500,000,000
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
Maximum [Member]
Line of Credit Facility, Leverage Ratio Covenant		3.75
Deferred Finance Costs, Net	$ 5,800,000	$ 5,800,000
Line of Credit Facility, Accordion Feature, Additional Revolving Credit Commitments Under Existing Loan Facility				$ 100,000,000
Amortization of Financing Costs	500,000	500,000	$ 500,000
Credit Facility Syndication, Number of Commercial Banks					11
Long-term Debt, Excluding Current Maturities	311,532,000	350,052,000
Letters of Credit Outstanding, Amount	3,700,000	4,400,000
Line of Credit Facility, Remaining Borrowing Capacity	184,800,000
Line of Credit Facility, Current Borrowing Capacity	131,900,000
Accumulated Amortization, Deferred Finance Costs	$ 4,000,000	$ 3,500,000
Line of Credit Facility, Interest Rate During Period	0.25%	0.30%