Note 8 - Property, Plant and Equipment and Construction-in-process - Activity in Construction and Development (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balance				$ 306,709					$ 306,709
Write off of unsuccessful exploration costs	$ (1,796)				$ (303)	$ (1,294)	$ (863)	$ (557)	(1,796)	$ (3,017)	$ (1,579)
Balance	293,542				306,709				293,542	306,709
Projects Under Exploration and Development [Member]
Balance				54,447				62,920	54,447	62,920	73,431
Cost incurred during the year									11,224	26,679	11,010
Write off of unsuccessful exploration costs									(1,796)	(3,017)	(1,579)
Transfer of projects under exploration and development to projects under construction										(32,135)	(19,942)
Balance	63,875				54,447				63,875	54,447	62,920
Projects Under Exploration and Development [Member] | Up-front Bonus Lease Costs [Member]
Balance				17,385				26,491	17,385	26,491	26,618
Cost incurred during the year										1,514	37
Write off of unsuccessful exploration costs									(367)	(380)	(164)
Transfer of projects under exploration and development to projects under construction										(10,240)
Balance	17,018				17,385				17,018	17,385	26,491
Projects Under Exploration and Development [Member] | Exploration and Development Costs [Member]
Balance				36,359				35,726	36,359	35,726	45,977
Cost incurred during the year									11,224	25,165	10,104
Write off of unsuccessful exploration costs									(1,429)	(2,637)	(1,415)
Transfer of projects under exploration and development to projects under construction										(21,895)	(18,940)
Balance	46,154				36,359				46,154	36,359	35,726
Projects Under Exploration and Development [Member] | Interest Capitalized [Member]
Balance				703				703	703	703	836
Cost incurred during the year											869
Write off of unsuccessful exploration costs
Transfer of projects under exploration and development to projects under construction											(1,002)
Balance	703				703				703	703	703
Construction in Progress [Member]
Balance				252,262				185,915	252,262	185,915	223,291
Cost incurred during the year									239,226	122,757	144,533
Transfer of projects under exploration and development to projects under construction										32,135	19,942
Transfer of completed projects to property, plant and equipment									(261,821)	(88,545)	(201,851)
Balance	229,667				252,262				229,667	252,262	185,915
Construction in Progress [Member] | Up-front Bonus Lease Costs [Member]
Balance				37,713				27,473	37,713	27,473	27,473
Cost incurred during the year
Transfer of projects under exploration and development to projects under construction										10,240
Transfer of completed projects to property, plant and equipment									(10,240)
Balance	27,473				37,713				27,473	37,713	27,473
Construction in Progress [Member] | Drilling And Construction Costs [Member]
Balance				202,211				150,467	202,211	150,467	187,545
Cost incurred during the year									231,926	116,247	140,977
Transfer of projects under exploration and development to projects under construction										21,895	18,940
Transfer of completed projects to property, plant and equipment									(235,194)	(86,398)	(196,995)
Balance	198,943				202,211				198,943	202,211	150,467
Construction in Progress [Member] | Interest Capitalized [Member]
Balance				$ 12,338				$ 7,975	12,338	7,975	8,273
Cost incurred during the year									7,300	6,510	3,556
Transfer of projects under exploration and development to projects under construction											1,002
Transfer of completed projects to property, plant and equipment									(16,387)	(2,147)	(4,856)
Balance	$ 3,251				$ 12,338				$ 3,251	$ 12,338	$ 7,975