Note 8 - Property, Plant and Equipment and Construction-in-process - Construction-in-process (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Construction-in-process	$ 376,555	$ 261,690
Projects Under Exploration and Development [Member]
Construction-in-process	84,637	70,958	$ 63,875	$ 54,447
Projects Under Exploration and Development [Member] | Up-front Bonus Lease Costs [Member]
Construction-in-process	17,018	17,018	17,018	17,385
Projects Under Exploration and Development [Member] | Exploration and Development Costs [Member]
Construction-in-process	66,916	53,237	46,154	36,359
Projects Under Exploration and Development [Member] | Interest Capitalized [Member]
Construction-in-process	703	703	$ 703	$ 703
Projects Under Construction [Member]
Construction-in-process	291,918	190,732
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	5,961	2,861
Projects Under Construction [Member] | Drilling And Construction Costs [Member]
Construction-in-process	$ 258,484	$ 160,398