Note 14 - Other Comprehensive Income ("OCI") - Components of AOCI (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other comprehensive income (loss), reclassification adjustment from AOCI on derivatives, net of tax	$ (2,784)	$ (10,597)	$ (9,608)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Other comprehensive income (loss), reclassification adjustment from AOCI on derivatives, before tax	(438)	(557)	(259)
Commodity Contract [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Other comprehensive income (loss), reclassification adjustment from AOCI on derivatives, net of tax	(2,511)	(10,255)	(9,448)
Commodity Contract [Member] | Fuel Expense [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Other comprehensive income (loss), reclassification adjustment from AOCI on derivatives, before tax	(4,065)	(16,674)	(15,313)
Commodity Contract [Member] | Income Tax Expense [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Other comprehensive income (loss), reclassification adjustment from AOCI on derivatives, before tax	1,554	6,419	5,865
Interest Rate Cap [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Other comprehensive income (loss), reclassification adjustment from AOCI on derivatives, before tax	(273)	(342)	(160)
Interest Rate Cap [Member] | Income Tax Expense [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Other comprehensive income (loss), reclassification adjustment from AOCI on derivatives, before tax	$ 165	$ 215	$ 99