Note 9 - Long-term Debt (Details Textual) $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	May 16, 2014 USD ($)
Credit Facility Syndication, Number of Commercial Banks				11
Line of Credit Facility, Maximum Borrowing Capacity Without Borrowing Base Requirement				$ 500.0
Line of Credit Facility, Accordion Feature, Additional Revolving Credit Commitments Under Existing Loan Facility				$ 100.0
Line of Credit Facility, Interest Rate During Period	0.25%	0.25%
Line of Credit Facility, Remaining Borrowing Capacity	$ 237.2
Line of Credit Facility, Current Borrowing Capacity	195.0
Debt Issuance Costs, Net	5.8	$ 5.8
Accumulated Amortization, Debt Issuance Costs	4.5	4.0
Amortization of Debt Issuance Costs	$ 0.5	$ 0.5	$ 0.5
Number of Letters of Credit, Outstanding	9	9
Letters of Credit Outstanding, Amount	$ 3.4	$ 3.7
Minimum [Member]
Line of Credit Facility, Fixed Charge Coverage Ratio Covenant				1.25
Maximum [Member]
Line of Credit Facility, Leverage Ratio Covenant				3.75
Line of Credit [Member]
Line of Credit Facility, Maximum Borrowing Capacity				$ 30.0
Amended Agreement [Member]
Line of Credit Facility, Expiration Date	May 16, 2014