Note 8 - Property, Plant and Equipment and Construction-in-process - Construction-in-process (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Construction-in-process	$ 261,690	$ 293,542
Projects Under Exploration and Development [Member]
Construction-in-process	70,958	63,875	$ 54,447	$ 62,920
Projects Under Exploration and Development [Member] | Up-front Bonus Lease Costs [Member]
Construction-in-process	17,018	17,018	17,385	26,491
Projects Under Exploration and Development [Member] | Exploration and Development Costs [Member]
Construction-in-process	53,237	46,154	36,359	35,726
Projects Under Exploration and Development [Member] | Interest Capitalized [Member]
Construction-in-process	703	703	$ 703	$ 703
Projects Under Construction [Member]
Construction-in-process	190,732	229,667
Projects Under Construction [Member] | Up-front Bonus Lease Costs [Member]
Construction-in-process	27,473	27,473
Projects Under Construction [Member] | Interest Capitalized [Member]
Construction-in-process	2,861	3,251
Projects Under Construction [Member] | Drilling And Construction Costs [Member]
Construction-in-process	$ 160,398	$ 198,943