Note 5 - Open Contract Positions Subject to Legally Enforceable Netting Arrangements - Amount of Derivative Asset and Derivative Liability Balances (Details) - Legally Enforceable Netting Arrangements [Member] - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Open Contract Gain Positions Subject to Legally Enforceable Netting Arrangements
Open Contract Loss Positions Subject to Legally Enforceable Netting Arrangements	(13,856)	(17,382)
Net Balance Subject to Legally Enforceable Netting Arrangements	$ (13,856)	$ (17,382)