Note 8 - Property, Plant and Equipment and Construction-in-process - Construction-in-process (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Construction-in-process	$ 293,542	$ 306,709
Projects Under Exploration and Development [Member]
Construction-in-process	63,875	54,447	$ 62,920	$ 73,431
Projects Under Exploration and Development [Member] | Up-front Bonus Lease Costs [Member]
Construction-in-process	17,018	17,385	26,491	26,618
Projects Under Exploration and Development [Member] | Exploration and Development Costs [Member]
Construction-in-process	46,154	36,359	35,726	45,977
Projects Under Exploration and Development [Member] | Interest Capitalized [Member]
Construction-in-process	703	703	$ 703	$ 836
Projects Under Construction [Member]
Construction-in-process	229,667	252,262
Projects Under Construction [Member] | Up-front Bonus Lease Costs [Member]
Construction-in-process	27,473	37,713
Projects Under Construction [Member] | Interest Capitalized [Member]
Construction-in-process	3,251	12,338
Projects Under Construction [Member] | Drilling And Construction Costs [Member]
Construction-in-process	$ 198,943	$ 202,211