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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Balance				$ 293,542					$ 293,542
Write off of unsuccessful exploration costs	$ (3)			(123)	$ (1,796)				(126)	$ (1,796)	$ (3,017)
Balance	261,690				293,542				261,690	293,542
Projects Under Exploration and Development [Member]
Balance				63,875				54,447	63,875	54,447	62,920
Cost incurred during the year									7,209	11,224	26,679
Write off of unsuccessful exploration costs									(126)	(1,796)	(3,017)
Transfer of projects under exploration and development to projects under construction											(32,135)
Balance	70,958				63,875				70,958	63,875	54,447
Projects Under Exploration and Development [Member] | Up-front Bonus Lease Costs [Member]
Balance				17,018				17,385	17,018	17,385	26,491
Cost incurred during the year											1,514
Write off of unsuccessful exploration costs										(367)	(380)
Transfer of projects under exploration and development to projects under construction											(10,240)
Balance	17,018				17,018				17,018	17,018	17,385
Projects Under Exploration and Development [Member] | Exploration and Development Costs [Member]
Balance				46,154				36,359	46,154	36,359	35,726
Cost incurred during the year									7,209	11,224	25,165
Write off of unsuccessful exploration costs									(126)	(1,429)	(2,637)
Transfer of projects under exploration and development to projects under construction											(21,895)
Balance	53,237				46,154				53,237	46,154	36,359
Projects Under Exploration and Development [Member] | Interest Capitalized [Member]
Balance				703				703	703	703	703
Cost incurred during the year
Write off of unsuccessful exploration costs
Transfer of projects under exploration and development to projects under construction
Balance	703				703				703	703	703
Construction in Progress [Member]
Balance				229,667				252,262	229,667	252,262	185,915
Cost incurred during the year									219,610	239,226	122,757
Transfer of projects under exploration and development to projects under construction											32,135
Balance	190,732				229,667				190,732	229,667	252,262
Transfer of completed projects to property, plant and equipment									(261,833)	(261,821)	(88,545)
Cost write off									(1,380)
Fair value of projects under construction acquired in a business combination									4,668
Construction in Progress [Member] | Up-front Bonus Lease Costs [Member]
Balance				27,473				37,713	27,473	37,713	27,473
Cost incurred during the year
Transfer of projects under exploration and development to projects under construction											10,240
Balance	27,473				27,473				27,473	27,473	37,713
Transfer of completed projects to property, plant and equipment										(10,240)
Cost write off
Fair value of projects under construction acquired in a business combination
Construction in Progress [Member] | Drilling And Construction Costs [Member]
Balance				198,943				202,211	198,943	202,211	150,467
Cost incurred during the year									219,610	231,926	116,247
Transfer of projects under exploration and development to projects under construction											21,895
Balance	160,398				198,943				160,398	198,943	202,211
Transfer of completed projects to property, plant and equipment									(261,443)	(235,194)	(86,398)
Cost write off									(1,380)
Fair value of projects under construction acquired in a business combination									4,668
Construction in Progress [Member] | Interest Capitalized [Member]
Balance				$ 3,251				$ 12,338	3,251	12,338	7,975
Cost incurred during the year										7,300	6,510
Transfer of projects under exploration and development to projects under construction
Balance	$ 2,861				$ 3,251				2,861	3,251	12,338
Transfer of completed projects to property, plant and equipment									(390)	$ (16,387)	$ (2,147)
Cost write off
Fair value of projects under construction acquired in a business combination