Note 9 - Senior Notes, Credit Facilities and Capital Leases (Details Textual)		3 Months Ended		9 Months Ended	12 Months Ended
	Apr. 12, 2013 USD ($)	Mar. 31, 2016 USD ($)	Mar. 31, 2015 USD ($)	Sep. 30, 2015	Dec. 31, 2014 USD ($)	Dec. 31, 2017	Dec. 31, 2015 USD ($)	Feb. 28, 2014 USD ($)	Apr. 30, 2013 USD ($)
Revolving Credit Facility [Member]
Long-term Line of Credit		$ 0
Line of Credit Facility, Maximum Borrowing Capacity								$ 100,000,000
Senior Notes [Member]
Debt Instrument, Face Amount	$ 500,000,000
Pension Contributions					$ 6,500,000
Debt Instrument, Unamortized Discount		6,500,000							$ 10,000,000
Debt Instrument, Interest Rate, Stated Percentage	6.875%
Interest Expense, Debt		3,800,000	$ 3,300,000
Interest Costs Capitalized		$ 5,200,000	$ 5,700,000
Debt Instrument, Redemption Price, Percentage				101.00%
Subsequent Event [Member]
Leverage Ratio						4
Proceeds from Issuance of Senior Long-term Debt	$ 490,000,000
Underwriting Discount On Senior Notes	2.00%
Leverage Ratio		5
Capital Lease Obligations		$ 15,642,000					$ 17,600,000
Capital Lease Obligations, Current		8,216,000					8,735,000
Capital Lease Obligations, Noncurrent		7,427,000					$ 8,841,000
Capital Leases, Future Minimum Payments Due		16,309,000
Capital Leases, Future Minimum Payments, Interest Included in Payments		$ 667,000